Mix Dry Ingredients: Start by adding flour, milk powder, salt, baking powder, and baking soda. Mix these ingredients together.
Cream Butter and Sugars: In a stand mixer with the paddle attachment, add butter, brown sugar, and sugar. Mix until well combined.
Add Egg and Vanilla: Mix in one egg and vanilla into the creamed butter and sugars.
Combine with Dry Ingredients: While mixing slowly, add in the flour mixture until the dough forms.
Add M&Ms: Incorporate M&M candies into the dough by mixing.
Shape and Chill: Shape the dough into a log, wrap it in plastic wrap, and cool in the refrigerator for 30 minutes.
Prepare for Baking: Cut the log into small pieces and roll the dough into balls.
Bake: Bake at 375°F for 10 minutes.