Cream together the unsalted butter, brown sugar, and granulated sugar in a large mixing bowl for 2-3 minutes until light and fluffy.

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

Beat the egg and vanilla extract into the creamed butter and sugar mixture until fully combined.

Gradually add the dry ingredients to the wet mixture, stirring gently and then fold in the mini marshmallows.

Chill the dough in the refrigerator for 30 minutes if too sticky to handle.

Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper. Drop spoonfuls of dough onto the sheet.

Bake for 10-12 minutes, rotating halfway through, until set and slightly puffed. Cool for 5 minutes before transferring to a wire rack.