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

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

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

Beat in the egg and vanilla extract until fully incorporated, scraping down the sides as needed.

Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined.

Gently fold in the chocolate chips, mini marshmallows, and marshmallow fluff if using.

Scoop the dough onto the prepared baking sheets, leaving 2 inches between each cookie.

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

Allow the cookies to cool on the baking sheets for about 5 minutes before transferring to a wire rack.

Enjoy your cookies once completely cooled.