Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a microwave-safe bowl, combine the unsalted butter and semi-sweet chocolate chips. Microwave in 30-second intervals, stirring until melted and smooth. Allow to cool slightly.
In a mixing bowl, whisk together the melted chocolate mixture, brown sugar, egg, and vanilla extract until well combined.
In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt.
Gradually fold the dry ingredients into the wet ingredients using a spatula until just combined.
Gently fold in the remaining chocolate chips until evenly distributed.
Scoop rounded balls of dough onto the prepared baking sheet, spacing them 2 inches apart.
Press a mini marshmallow into the center of each cookie dough ball. Optionally, sprinkle with finely chopped chocolate.
Bake the cookies for 10-12 minutes, until the edges are set but the centers are slightly soft.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
