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

In a medium bowl, whisk together all-purpose flour, cocoa powder, baking soda, and salt.

In a larger bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.

Add the egg and vanilla extract, mixing until well combined.

Gradually incorporate the dry ingredients into the wet mixture, mixing on low to avoid overmixing.

Gently fold in the chocolate chips, mini marshmallows, and marshmallow fluff until evenly distributed.

Scoop the dough onto the prepared baking sheets, spacing them about 2 inches apart.

Bake for 8-10 minutes until the edges are set and the centers are soft.

Cool on the baking sheet for 5 minutes before transferring to a wire rack.