Preheat the oven to 350°F (177°C).
Line 12-cup muffin pan with paper liners and lightly spray with non-stick spray. (See note 2)
In a large bowl, mash thawed bananas with a fork or wooden spoon.
Add sugar, melted butter, egg and vanilla extract, and mix everything until combined.
Then add flour, baking soda and salt. Mix to combine.
Add walnuts and chocolate chips, if using.
Divide the batter evenly into the prepared muffin cups.
Bake for 21-23 minutes, or until the inserted toothpick comes out clean. Cool completely before serving.
Store muffins, covered, on the counter for up to 3 days.