Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper or silicone baking mats.
In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chocolate chips and mini marshmallows until evenly distributed.
Drop tablespoon-sized balls of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Bake in the preheated oven for 10-12 minutes, or until the edges are set and the marshmallows are lightly browned.
Allow the cookies to cool on the baking sheets for 5 minutes before transferring to wire racks to cool completely. Enjoy!