Preheat oven: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

Mix dry ingredients: In a medium bowl, whisk together the flour, cocoa powder, espresso powder, baking soda, and salt.

Cream butter and sugars: In a large mixing bowl, use an electric mixer to cream the softened butter, brown sugar, and granulated sugar together until light and fluffy (about 2–3 minutes).

Combine wet and dry: Mix in the egg and vanilla extract until well combined. Add the dry ingredients to the wet mixture and mix until just combined. (Don’t overmix.) Fold in the M&Ms until combined.

Chill: Cover and refrigerate the cookie dough for 2 hours or until chilled.

Scoop the dough: Use a ¼-cup cookie scoop to portion the dough onto the prepared baking sheets, spacing them about 2 inches apart. Gently press some M&Ms into the tops of each dough ball.

Bake: Bake for 9–11 minutes, or until the edges are set and the centers look slightly underbaked (they’ll firm up as they cool).

Cool: Let cookies cool on the pan for 5 minutes, then transfer to a wire rack to cool completely.