Add butter and both sugars to the bowl of a stand mixer fitted with the paddle attachment and beat until smooth, about 3 minutes.
Add the egg and vanilla and beat again until combined, scraping down the sides of the bowl if necessary.
Add the flour, baking powder, salt, and baking soda and beat again until a thick, uniform dough forms.
Fold in the white chocolate chips, dark chocolate chips, and half the M&Ms. Cover and chill the dough for 30 minutes in the fridge.
Meanwhile, preheat the oven to 350℉ and line a large baking sheet or 2 medium baking sheets with parchment paper.
Using a 2-inch cookie scoop, form 18 balls of dough and place them at least 3 inches apart on the baking sheet. Flatten each slightly with your hand, and press 4-5 of the M&Ms on the tops of each cookie ball.
Bake in the preheated oven for 12-14 minutes, or until just beginning to brown around the edges. Allow to cool, then serve!