Preheat oven to 350 degrees. Line baking sheet with parchment paper or silicone baking mat and set aside.
In large bowl, cream butter and sugars. Stir in egg and vanilla. Scrape sides and whip until light and fluffy. Stir in all dry ingredients until just incorporated. Scrape sides and mix again briefly.
Scoop onto prepared baking sheets and press mini M&M's into the tops of each cookie dough ball. Bake 8-9 minutes. Cool cookies 5 minutes on hot baking sheet to finish baking, then remove to cool completely. Store in an airtight container for up to 3 days.