Prepare the Dough : Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside. In a large mixing bowl, cream the butter and sugar together until light and fluffy (about 2-3 minutes). Beat in the eggs one at a time, followed by the vanilla extract, until fully incorporated.

Combine Wet and Dry Ingredients : Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a smooth, chocolatey dough forms.

Swirl in the Marshmallow Fluff : Gently fold the marshmallow fluff into the dough, creating swirls. Be careful not to overmix—the goal is to maintain visible streaks of marshmallow.

Shape and Bake the Cookies : Using a cookie scoop or tablespoon, drop rounded spoonfuls of dough onto the prepared baking sheets, spacing them about 2 inches apart. Bake for 10-12 minutes, or until the edges are set and the centers are slightly puffed. The cookies will continue to set as they cool. Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.