Preheat oven to 350°F. Line two baking sheets with parchment paper.
In a bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
In a separate bowl, using a hand mixer or stand mixer fitted with the paddle attachment, beat butter and both sugars for 2 minutes, until light and fluffy. Add egg and vanilla extract, mixing until combined, scraping the sides of the bowl as needed. On low speed, add in flour mixture and mix until just combined.
Gently stir in M&M's and chocolate chips. I usually add about 1 generous cup into the batter, and the rest get pressed on top of the cookies before baking.
Place the sprinkles in a shallow bowl.
Scoop balls of cookie dough, between 1½ and 2½ tablespoon portions (a little smaller or a little larger than a golf ball). Gently press the top of the dough balls into the bowl of sprinkles, then press some of the remaining M&M's onto the top of each mound of dough. Place dough onto prepared baking sheets leaving an inch or two for spreading.
Bake for 8-11 minutes (see note), until the edges of the cookies are set but the center is still slightly underdone; this will create a soft cookie.
Place the baking sheets on wire racks and allow the cookies to cool completely. They will firm up as they cool. Optional: for a slightly thicker cookie, you can gently press the edges of each cookie inward with the back of a spoon (just a bit!) while they are still warm.