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

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

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

Add egg and vanilla extract to the mixture and beat until smooth.

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

Fold in chocolate chips and mini marshmallows until evenly distributed.

Scoop tablespoon-sized portions of dough onto prepared baking sheets, spaced about 2 inches apart.

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

Let cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.