Adapted from the Joy of Baking
3/4 cup (1 1/2 sticks) butter, at room temperature
2/3 cup sugar
2/3 cup packed brown sugar
1 egg yolk
1 1/2 teaspoons vanilla extract
2 cups flour
1 1/4 teaspoon baking soda
1/4 teaspoon salt
1 cup M & M's
Preheat oven to 350º and line cookie sheets with parchment paper. Set aside.
Beat the butter and sugars till light and fluffy. Add the egg and egg yolk and mix till combined. Add vanilla and mix. Sprinkle baking soda and salt over the butter mixture and mix till combined. Add and mix in flour. If your kitchen is warm, you may want to refrigerate dough up till firm...up to an hour.
Scoop out 2 tablespoons full of dough per cookie and roll into balls. Place 6-8 dough balls on each cooking sheet, flatten slightly with palm of your hand and cover top with 6-8 M & M's.
Bake for about 12 minutes or till light golden brown. Let cool a few minutes on cookie sheet before removing to cooling rack.
Makes about 30 cookies (results may vary depending on size you make your cookies).