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

Whisk together the flour, cocoa powder, baking soda, and salt in a medium bowl.

Beat together the butter and sugar until light and fluffy, about 3–4 minutes.

Add the egg and vanilla to the butter-sugar mixture and beat until well combined.

Gradually mix the dry ingredients into the wet mixture until a dough forms.

Fold in the marshmallows gently until evenly distributed.

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

Bake for 10–12 minutes, rotating trays halfway through. Let cool for 5 minutes before transferring to a wire rack.