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

In a stand mixer, cream together the room-temperature unsalted butter with granulated and brown sugars for about 2-3 minutes.

Add the egg and vanilla extract, mixing until just combined.

In a separate bowl, sift together the all-purpose flour, cocoa powder, baking powder, and a pinch of salt.

Fold the dry ingredients into the wet mixture until no dry flour remains visible.

Carefully fold in the chocolate chips and mini marshmallows into the dough.

Using a cookie scoop, portion out the cookie dough onto the lined baking sheet, spacing them about 2 inches apart.

Bake for 10-12 minutes until edges are set; centers should still appear soft.

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