Line a baking sheet with parchment paper and set it aside.
Cream the softened butter, light brown sugar, and granulated sugar together for 3 minutes until light and fluffy.
Mix in the egg, egg yolk, and vanilla extract until just combined.
Mix in the flour, salt, baking soda, and baking powder until almost combined and there are still some dry flour streaks. Then fold in the chocolate chips and M&Ms.
Scoop the dough into 2 tablespoons balls and place them on the prepared baking sheet. For cookies that bake evenly, roll each one between your palms until round. Then press more M&Ms and chocolate chips to the tops of each cookie dough ball.
Chill the tray of cookie dough balls in the freezer for 3 hours minimum, overnight is best for the best flavor and texture.
Then preheat the oven to 350 F/180 C. Bake the cookies spaced 2-3 inches apart on a parchment-lined baking tray for 10-12 minutes, or until the edges are light golden brown.
Let the tray cool on a wire cooling rack before removing them, then enjoy!