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

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

In a large bowl, cream the unsalted butter, granulated sugar, and brown sugar until light and fluffy.

Beat in the egg and vanilla extract until fully combined.

Gradually mix in the dry ingredients until just combined.

Fold in the semi-sweet chocolate chips evenly throughout the dough.

Gently stir in the mini marshmallows, creating swirls in the dough.

Drop rounded tablespoons of dough onto prepared baking sheets, spaced 2 inches apart.

Bake for 8-10 minutes, watching to ensure edges are set and centers remain slightly soft.

Cool cookies on baking sheet for 5 minutes, then transfer to wire rack to cool completely.