In a large mixing bowl, cream together the unsalted butter, brown sugar, and granulated sugar until light and fluffy, about 3 to 5 minutes.

Add the eggs one at a time, mixing well after each addition.

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

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

Fold in the mini marshmallows gently with a spatula. Chill dough for about 30 minutes if too sticky.

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

Drop spoonfuls of dough onto prepared sheets, leaving space between cookies. Bake for 10-12 minutes.

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