Preheat the oven to 375° and line a cookie sheet with parchment or a silicone mat.

In a medium bowl, cream the butter, brown sugar and granulated sugar together. Beat very well, about 45 seconds. Add the egg yolk and vanilla and beat until combined.

In a small bowl, whisk together remaining ingredients (except M&Ms) and add it in 2 batches to the creamed butter, beating just to combine.

Stir in the chopped M&Ms. Make 12 dough balls (roll them in your hand), space them evenly on the cookie sheet and bake for 9-10 minutes.

Remove from oven, and let cool for 1 minute on the baking sheet before moving them to a wire rack to cool completely.