Line a baking sheet with parchment paper and set aside.
In a medium-sized bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set the bowl aside.
In the bowl of a stand mixer, cream the butter and sugars together for a few minutes, until they are light and fluffy. Beat in the egg and add the vanilla extract.
Little by little, add the dry ingredients to the wet, on low speed. Scrape down the sides of the bowl as necessary. Add the mini chocolate chips. Turning the speed up a bit higher helps to incorporate them throughout the thick dough.
Next, you’re going to want to scoop and chill the cookie dough. Scoop and roll the cookie dough into 1.5 tbs-sized balls. Place them on the prepared cookie sheet. Cover the dough balls tightly with plastic wrap and chill it for at least 3 hours, and up to overnight.
Once you’re ready to bake the cookies, preheat the oven to 350 degrees F and line another baking sheet with parchment paper. Move half of the dough balls over to the other cookie sheet and re-position all of the dough so that it is evenly spaced, leaving a couple of inches between each dough ball.
Bake the cookies for 9-10 minutes. The cookies will look pretty soft in the middle, and that’s okay. Remove them from the oven and gently press the M&Ms into the warm cookies. Allow the cookies to cool on the cookie sheets for 5 minutes or so before removing them to a wire rack to finish cooling completely.