In a small bowl, melt butter and 2/3 cup of the chocolate chips together in the microwave. Heat for 30 seconds and stir. Then continue in 15-second bursts, stirring in between until smooth. Set aside to cool slightly for about five minutes.
Mix brown sugar, egg, and vanilla in a large bowl until just combined. Beat in the cooled chocolate mixture.
Mix in flour, cocoa powder, baking powder, and salt, until just combined.
Cover and chill for 45 minutes until the dough is firm enough to handle.
Preheat oven to 325 degrees Grease or cover two cookie sheets with parchment paper.
Scoop dough and roll into 1-inch balls. Place them on the baking trays 2-inches apart.
Bake for 7 minutes before removing the cookies from the oven. Place 2-3 chocolate chips in the center of each cookie and top with a mini marshmallow. Press the marshmallow firmly into the chocolate and place the tray back into the oven.
Bake cookies for an additional 4 minutes and remove pan from the oven. While the cookies are still hot, sprinkle with chopped chocolate, if so desired.