Prepare the Dough

1. Preheat Oven: Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
2. Cream Butter and Sugars: In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar until creamy.
3. Add Eggs and Vanilla: Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
4. Combine Dry Ingredients: In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
5. Mix Dry and Wet Ingredients: Gradually add the dry mixture to the wet mixture, stirring until just combined.
6. Fold in Marshmallows and Chocolate Chips: Gently fold in the mini marshmallows and chocolate chips until evenly distributed.

Shape and Bake the Cookies

1. Scoop the Dough: Using a cookie scoop or tablespoon, drop dough onto the prepared baking sheets, spacing them about 2 inches apart.
2. Swirl the Marshmallows: With the back of a spoon, gently swirl the marshmallows into the tops of the cookie dough balls for a decorative touch.
3. Bake Cookies: Bake in the preheated oven for 12-15 minutes, or until the edges are set and the centers look slightly soft.
4. Cool on Baking Sheets: Allow cookies to cool on the baking sheets for 5 minutes before transferring them to wire racks to cool completely.