Prepare the Dry Ingredients : In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.

Make the Cookie Dough : In a large mixing bowl, beat the butter and sugar together until light and fluffy, about 2-3 minutes. Add the eggs one at a time, mixing well after each addition. Mix in the vanilla extract. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Add the Marshmallow Swirl : Drop dollops of marshmallow fluff into the cookie dough. Use a spatula to gently swirl the marshmallow fluff into the dough. Be careful not to overmix; you want distinct swirls of marshmallow.

Scoop and Shape : Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper. Scoop tablespoon-sized portions of dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake the Cookies : Bake for 8-10 minutes, or until the edges are set but the centers remain soft. Remove the cookies from the oven and allow them to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.