Preheat the oven to 350 F. Prepare a baking sheet with parchment and set aside.
In a large bowl combine butter and sugars. Beat on low with an electric hand mixer or in a bowl of a stand mixer until smooth and creamy. Add eggs and vanilla. Mix on low until combined. Set aside.
In a separate large bowl, combine dry ingredients. Whisk to combine and add to wet ingredients gradually. Add ½ or 1 cup at a time, beat on low until combined, and then continue with the next batch.
Mix until ingredients are almost combined, a few lumps here and there are fine.
Stir in M&Ms. The batter will be thick, and a little sticky. Chill the dough in the freeze for 30 minutes to 1 hour in the fridge.
The batter will resemble playdough and should not be sticky after chilling. You should be able to roll small balls easily. If it is too frozen at this point, wait until it is room temp until it’s softened slightly. Scoop 2 tablespoonfuls of batter, rolling it into about 2-inch balls.
Arrange the cookie balls on a baking sheet lined with parchment paper at least 2 inches apart. Don’t press down!
Bake for 10-12 minutes. The cookies will have a soft center inside. Once they cool down they will set well and the texture inside remains soft, chewy, and buttery.
Let the cookies stand in the pan for 5-10 minutes before transferring them to a wire rack to cool completely.
Don’t substitute sugars. Both sugars should be used to get the best buttery and soft cookie. You can make these cookies only with white sugar, they will still taste good but there will be a difference.