To cook potatoes: Wash and peel potatoes, if desired. Cut into about 2-inch pieces and place them in a large pot, filled with cold water. Add salt and garlic powder.
Bring it to a boil over medium high heat, and then reduce heat to medium to gently simmer the potatoes. Cook until potatoes are nice and tender, about 25 minutes.
Carefully drain the water.
Squeeze out roasted garlic and mash them into a smooth paste.
To mash the potatoes: As soon as you drain the potatoes, stir in half of the hot milk, butter, roasted garlic and mayo. (TIP: It's important to mash the potatoes right away when they're hot to prevent gluye texture.)
Using a potato masher, mash the potatoes until nice and smooth (or chunky, however you like it). Keep adding the rest of the milk (or more) until desired consistency.
Season with freshly ground black pepper and additional salt, if needed. Serve immediately, or put it in a warm oven to keep it hot. Or place it in a slow cooker set to 'keep warm' setting.