All ingredients ready? Let's begin!
Sift the flour, baking powder, and salt together in a medium bowl. Set aside.
In a large bowl using a hand mixer or stand mixer with a paddle attachment, beat softened butter until creamy. About 1 minute.
Add brown sugar and granulated sugar. Beat on medium-high speed until combined and creamy. Scrape down the bottom and sides of the bowl as needed.
Beat in egg, egg yolk, and vanilla extract.
Turn the speed down to low and gradually add the flour mixture. Mix until combined.
Remove mixer and stir in M&Ms with a wooden spoon. Or beat on low speed for 20 seconds or until combined.
Cover cookie dough with plastic wrap and chill in the fridge for 2 hours.
Preheat oven to 350°F and line baking sheets with parchment paper.
Roll cookie dough into balls of about 1.5 tablespoons each, and place 3 inches apart on baking sheets.
Bake for 11-12 minutes or until slightly browned on the edges. The centers should look soft.
Remove from the oven and let cookies cool for 5 minutes. Then transfer to a wire rack and let them cool completely.