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.
½ cup butter
Whisk the butter until it is completely melted.
Add the sugars; mix until well-combined.
¾ cup brown sugar, ¼ cup granulated sugar
Stir in vanilla and egg until incorporated.
1 teaspoon vanilla extract, 1 large egg
Add the flour, cocoa powder, baking soda, and salt. Please read the recipe note about properly measuring flour.
1 ⅓ cups all-purpose flour, ⅓ cup unsweetened cocoa powder, ½ teaspoon baking soda, ¼ teaspoon salt
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.
½ cup mini semisweet chocolate chips, 1 cup plain 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.
Video
Notes
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.