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

Using a hand mixer or stand mixer, cream together the butter, brown sugar, and granulated sugar for about 2 minutes until light and fluffy. Add the eggs and vanilla extract and mix until combined.

In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt. Add the dry ingredients to the butter mixture and mix on low speed until just combined. Be careful not to overmix. Fold in the chocolate chips.

Using a 2-ounce cookie scoop or a 1/4 cup measure, scoop dough balls and place 6 cookies at a time onto the prepared baking sheet, spacing them about 2 inches apart.

Bake for about 12 minutes. It is better to underbake slightly than overbake. Let the cookies cool on the baking sheet for 2-3 minutes, then transfer to a wire rack to cool completely.

Place the mini marshmallows in a microwave-safe bowl. Microwave for 45 seconds to 1 minute.

Remove the bowl and stir the marshmallows. Microwave in additional 15-second increments until fully melted and smooth, stirring between each.

Let the melted marshmallows cool slightly until they are warm but not too hot to handle. If they start to harden, reheat in the microwave in short bursts until melted again.

Test a small amount on your fingers to ensure it's not too hot. Using both hands, pinch a small amount of the melted marshmallow between your thumb and forefinger.

Pull your fingers apart to create thin strands of marshmallow. Stretch these strands over each cookie, rubbing your fingers around the edge to help the marshmallow stick and separate from your fingers.

Repeat with each cookie, reheating the marshmallows as needed to keep them pliable.