Preheat the oven to 350°F. Line a sheet pan with parchment paper or a silicone mat.

Combine flour, cocoa powders, salt, and baking soda in a medium mixing bowl. Whisk well to combine. Set aside.

In a large mixing bowl, use mixer to beat cool room temperature butter until smooth and slightly aerated, about 1-2 minutes. Scrape down the bowl, add both sugars, and beat again on medium to high speed until it is fully incorporated, lightened in color, and has about doubled in size. It might not quite smooth out, and will still be quite grainy. Be patient here — we need to incorporate quite a bit of air to help the cookies spread. This step can take 2-3 minutes. Scrape down the bowl halfway through.

Add the egg and vanilla and beat again on medium to high speed light and fluffy, about 60-90 seconds. Scrape down the bowl.

Sift the dry ingredients into the wet ingredients. Press any lumps of cocoa powder through the mesh to break them up. If any salt doesn't fit through the mesh at the end, that's fine, dump it into the bowl.

Run the mixer on very low speed until the dry ingredients are hydrated but still fairly crumbly. There will still be some dry powdery bits in the bottom of the bowl. The remaining powdery bits will incorporate when you stir in the mini M&Ms so you don't overmix the dough.

Add the mini M&Ms (reserve about ⅛ cup for topping the cookies) and switch to a spatula. Fold the cookie dough over itself to incorporate the M&Ms. It will seem like there are too many M&Ms at first, but I promise there is no such thing. Just keep folding and pressing the cookie dough over itself with the spatula until the mini M&Ms are evenly incorporated. Do not overmix.

Use a #20 cookie scoop to scoop the dough, scraping any excess dough off on the edge of the bowl for precise measurements. Roll the dough into balls, then flatten them to about ½" thick. Arrange on a sheet pan about 3" apart. Press additional mini M&Ms into the tops of the cookies.

Bake for 10-11 minutes. The cookies will look puffy and slightly underbaked when they're done. Let them rest on the sheet pan for 5 minutes, then transfer to a cooling rack to finish cooling. Optional: Use a 3" circle cookie cutter or ring to scoot the cookies into perfectly round shapes immediately after removing from the oven.