Preheat your oven to 350°F (175°C) and line a baking sheet 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 together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes.
Add the vanilla extract and egg to the butter mixture, mixing until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the chocolate chips and mini marshmallows.
Drop rounded tablespoons of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes or until the edges are set but the centers are still soft. Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.