Preheat the oven to 350 F. Line 2 baking sheets with parchment paper or silicone baking mats.
Microwave the butter in a large, microwave-safe bowl for about 40 seconds. Butter should mostly melted.
Whisk the butter until it is completely melted.
Add the sugars; mix until well-combined.
Stir in vanilla and egg until incorporated.
Add the flour, cocoa powder, baking soda, and salt. Please read the recipe note about properly measuring flour.
Mix dough until just combined. Dough should be soft and a little sticky but not overly sticky.
Stir in mini chocolate chips and M&Ms.
Scoop out 1.5 tablespoons of dough (medium cookie scoop) and place 2 inches apart on baking sheet.
Bake for 7-10 minutes, or until cookies are set. They will be puffy and still look a little underbaked in the middle.
Flour: Be sure to properly measure the flour. To measure the flour, stir or sift the flour to break it up. Then lightly spoon into the measuring cup and level. I don't recommend scooping as it can pack in the flour.
M&Ms: I recommend using plain (not peanut) M&Ms in these cookies because I've had better luck with them in the cookies.
Baking: Slightly underbaking the cookies helps to keep them soft.