In a large bowl, melt your butter then set aside to cool for 5 minutes. Add the brown sugar and granulated sugar and whisk to combine.
170 grams unsalted butter, 160 grams brown sugar, 160 grams granulated sugar
Add the egg and vanilla extract and whisk well until combined. Add the baking soda, baking powder, and salt and whisk until combined.
1 egg, 1 tablespoon vanilla extract, ½ teaspoon baking soda, ¾ teaspoon baking powder, ½ teaspoon salt
Add the flour and dark cocoa powder and fold until almost incorporated and there are only a few streaks of flour remaining. Add the mini chocolate chips and M&Ms and fold until evenly incorporated throughout the dough and the streaks of flour are gone.
219 grams all purpose flour, 55 grams dark (dutch processed) cocoa powder, 90 grams mini chocolate chips, 100 grams regular M&Ms
Cover the dough tightly with plastic wrap and chill in the fridge for at least two hours but up to 24 hours.
Once ready to bake, preheat your oven to 350F and line a baking sheet with parchment paper. Scoop the dough into ¼ cup sized balls (about 75 grams) and space them on the baking sheet 2-3 inches apart. Add more chocolate chips and M&Ms to the tops of the cookies if desired.
Bake for 11-13 minutes until the edges are set and the centers are puffed up, then remove from the oven. Let cool for 5 minutes on the pan before transferring to a cooling rack.
Let cool for an additional 15 minutes and enjoy slightly warm, or let cool completely. Store in an airtight container at room temperature.