Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper.

In a stand mixer or a medium bowl with an electric mixer, beat the butter with the regular and brown sugar on medium speed until creamy. Add the egg and vanilla and beat on low speed just until combined; do not overmix.

In a small bowl, whisk together the flour, baking soda, and salt.Add the flour mixture to the butter mixture and mix just until combined. (For tender cookies, don't overmix.) Stir in the mini and regular m&ms.

Roll the dough in balls that are about 2 level tablespoons each. Arrange on the prepared baking sheet with plenty of room for spreading. (I bake 6 cookies on a large baking sheet.) Bake just until the cookies are puffed and barely starting to brown around the edges, about 9 to 11 minutes

Cool the pan on a wire rack and remove cookies with a spatula after cooling for 2 minutes. Makes about 20 cookies.