Monster Cookie Bars

5 from 1 vote

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It’s impossible to resist these peanut butter oatmeal monster cookie bars, loaded with chocolate chips and M&Ms. There’s no flour, and they’re made in just one bowl!

Why you’ll love this recipe: 

Monster cookie bars are my go-to dessert when I need to bake for a crowd or send in a school treat, especially if it’s at the last minute! This large batch recipe yields 4 dozen bars and only takes 30 minutes. 

Not to mention, the soft and chewy texture of the bars is to die for! 

  • Kids love the bright and colorful M&Ms.
  • Feel free to play around with the classic recipe, and use holiday-themed M&Ms and different chocolate chip flavors! 
  • Easily make this recipe gluten-free by using gluten-free oats.   

If you’re craving something even more rich and decadent, you need to try my salted caramel chocolate chip cookie bars, too! 

A stack of five monster cookie bars.

Key Ingredient Notes:

  • Unsalted butter – This gives the dough a rich flavor and moisture, especially the rolled oats. The oats will soak up the butter, making them nice and soft.  
  • Brown sugar and granulated sugar – When used together, these add sweetness to the bars while keeping them moist. Use light or dark brown sugar, whatever you prefer. 
  • Eggs – Room-temperature eggs are ideal, because they blend more seamlessly into the dough, improving the overall texture.  
  • Creamy peanut butter – Smooth (not crunchy) peanut butter is best. Peanut butter isn’t added just for flavor; it’s a crucial binding ingredient contributing to the soft and chewy texture! Try to avoid natural peanut butter as it tends to separate and can cause your bars to be runny. 
  • Old-fashioned rolled oats – These give the bars structure, holding the ingredients together while enhancing the texture. 
  • Baking soda – This helps the cookie bars rise. Make sure it’s fresh! 
  • Chocolate chips – I recommend using semi-sweet chocolate chips. Since the dough contains added sugar and M&Ms, a sweeter chocolate chip, like milk chocolate, isn’t necessary. 
  • M&Ms – These add color and sweetness and are just plain fun! I prefer regular-sized M&Ms, but mini M&Ms also work. 
Ingredients needed to make monster cookie bars.

How to make this recipe:

Before you get started, preheat your oven to 350°F, and line a baker’s half baking sheet with parchment paper.

  • Cream the butter, brown sugar, and granulated sugar on medium-high speed in an electric mixing bowl fitted with the paddle attachment. Beat until nice and fluffy, about 3 minutes. 
  • Add the eggs, peanut butter, and vanilla extract, and continue mixing on medium-high speed until well combined, about 2 minutes. 
  • Add the rolled oats and baking soda, and continue mixing for about 1 more minute. 
  • Stir in the chocolate chips and M&Ms. 
Four images showing the process of combining ingredients to make monster cookie bar dough.
  • Transfer the monster cookie dough to your prepared baking sheet. 
  • Spread the mixture out into an even layer on the baking sheet using a silicone spatula.

3. Bake and cut the bars

  • Bake the bars for 15 minutes or until they’re golden brown and just set in the middle.
  • Don’t overbake! They’ll continue to set and firm up as they cool.  
  • Let the bars cool completely before slicing them into 48 equal-sized squares.
Two images showing unbaked and baked monster cookie bars in a pan.


If you want to make these bars extra fancy, feel free to top them with royal icing or powdered sugar icing, and add your favorite toppings! 

Tips for Success:

  • Make thicker bars using a smaller baking pan. If you’d prefer extra-thick cookie bars, use a 9×13-inch baking pan instead of a half baking sheet. Keep in mind that a smaller pan will result in fewer bars and require a longer baking time of 18 minutes instead of the usual 15 minutes.
  • Leave some overhang when lining the baking sheet with parchment paper. Extra parchment around the edges will give you handles to grab onto, making the bars so much easier to remove from the pan! 
  • Spread the dough out into an even thickness. An even layer of dough ensures the bars cook consistently throughout. Use a silicone spatula or your hands to press the dough firmly into the pan.    
  • Don’t overbake! Overbaking can result in dry, crumbly bars that fall apart. Trust me, pull the bars out of the oven right at 15 minutes (or 18 minutes for a smaller pan). They’ll finish baking as they cool.  
  • Plan for at least 1 hour of cooling time. For hand-held cookie bars that don’t fall apart, it’s essential to wait until they’re completely cooled before cutting them into squares.  
Sliced flourless monster cookie bars with one sticking up.

Make-Ahead Tips:

If you’re trying to minimize cleanup on baking day, consider preparing your dough in advance.

  • Prepare the cookie dough as directed in the recipe. 
  • Cover the prepared dough tightly in plastic wrap, and store it in the fridge for 2-3 days. 
  • Let the cookie dough sit at room temperature for at least 30 minutes before spreading it in the baking pan. This will soften the dough a bit, making it much easier to work with. 
  • Follow the remaining recipe steps when you’re ready to bake! 

Storing Tips:

  • Store cooled monster cookie bars in an airtight container on the counter for up to 1 week. I’m sure you’ll have them eaten up long before that!  
  • Cookie bars are freezer-friendly! Wrap each cookie bar tightly in plastic wrap to prevent freezer burn. Then, place the bars in a plastic ziplock freezer bag for extra protection. Freeze for up to 3 months.  
  • Let frozen bars thaw at room temperature before serving. 
A stack of three flourless monster cookie bars.
What size baking pan should I use to make monster cookie bars?

Most cookie bars are baked on a baker’s half baking sheet, measuring 17.9 x 12.9 x 1 inches. A baking sheet with a rim is necessary because the cookie bars will rise. Alternatively, a 9×13-inch baking dish also works. You’ll just end up with thicker bars that need a few extra minutes in the oven. 

What else can I add to monster cookie bars?

Besides M&Ms and chocolate chips, you’ll find monster cookie bars containing pretzel pieces, walnuts, peanut butter cups, and even leftover Halloween candy! Feel free to have fun with your mix-ins!  

Are my cookie bars supposed to look gooey?

When the bars are fresh out of the oven, they may appear gooey, especially towards the middle of the pan. A soft-looking center is normal and doesn’t mean a longer baking time is needed. They’ll firm up as they cool down.  

Can I make this recipe nut-free? 

Yes, feel free to swap out the peanut butter with any other nut or seed butter such as almond butter, cashew butter, or sunflower butter to cater to different allergies. The flavor will change slightly, depending on what you use, but the bars will still be tasty! 

Sliced monster cookie bars with one square sticking up.
5 from 1 vote

Monster Cookie Bars

Prepare these chocolate peanut butter monster cookie bars in just 30 minutes, without flour and using only one bowl!
Prep: 15 minutes
Cook: 15 minutes
Total: 30 minutes
Servings: 48 servings
Yields: 48 squares


  • 1 stick (113 g) unsalted butter at room temperature
  • ¾ cup (150 g) brown sugar Note 1
  • ¾ cup (150 g) granulated sugar
  • 3 large eggs at room temperature
  • 1 ½ cups (400 g) creamy peanut butter
  • 1 teaspoon pure vanilla extract
  • 4 ½ cups (400 g) old-fashioned rolled oats
  • 2 teaspoons baking soda
  • ¾ cup (120 g) semi-sweet chocolate chips
  • 1 cup (200 g) M&Ms



  • Preheat the oven to 350°F (177°C). Line a baker’s half baking sheet with a parchment paper. (You may use 9×13 baking pan as well for thicker bars.)
  • In a mixing bowl with paddle attachment, cream butter, brown and granulated sugars on medium high speed until nice and fluffy, about 3 minutes.
    1 stick unsalted butter, ¾ cup brown sugar, ¾ cup granulated sugar
  • Add eggs, peanut butter, and vanilla extract. Mix on medium high speed until well combined, about 2 minutes.
    3 large eggs, 1 ½ cups creamy peanut butter, 1 teaspoon pure vanilla extract
  • Add rolled oats and baking soda and mix until well combined, another minute or so.
    4 ½ cups old-fashioned rolled oats, 2 teaspoons baking soda
  • Stir in chocolate chips and M&Ms.
    ¾ cup semi-sweet chocolate chips, 1 cup M&Ms
  • Spread the mixture into an even layer in the prepared baking sheet.
  • Bake for 15 minutes (18 minutes for 9×13 baking pan). Don’t overbake! Cool completely.

Tips & Notes

Note 1: Both light and dark brown sugar works. 
Note 2: Feel free to use any other nut or seed butter such as almond butter, cashew butter, or sunflower butter to keep this recipe peanut-free. 
Note 3: Look for oats that are certified gluten-free to keep this recipe gluten-free-friendly. 
Note 4: Baker’s half baking sheet size is 17.9 x 12.9 x 1 inches. 
Storing Tips: 
– Store leftover monster cookie bars in an airtight container at room temperature for up to 1 week. 
– Freeze your monster cookie bars for up to 3 months. Wrap each bar tightly in plastic wrap, and place them in a ziplock freezer bag. 
– Thaw frozen bars at room temperature before serving. 


Servings: 1 serving
Calories: 144kcal
Carbohydrates: 18g
Protein: 4g
Fat: 7g
Sugar: 11g
Sodium: 89mg
Course: Dessert
Cuisine: American

Hi, I’m Shinee!

Welcome! I’m so happy you’re here! I believe anyone can cook restaurant-quality food at home! And my goal is to help you to become a confident cook with my easy-to-follow recipes with lots of tips and step-by-step photos.

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