Melt the butter and 12 ounces of semi-sweet chocolate chips in a medium sauce pan over medium heat. Stir constantly until melted. Remove the pan from heat and let cool for 5 minutes.

Combine the sugars, eggs, and vanilla extract in a large bowl. Beat with whisk or electric beater until evenly combined.

Beat in the cooled chocolate until evenly combined.

Add in the cocoa powder, flour, baking powder and salt. Mix until all ingredients are mixed in. Cover bowl with saran wrap and refrigerate for 2 hours.

Preheat the oven to 325. Line cookie sheets with parchment paper or aluminum foil. Scoop up a tablespoon of dough and roll it in hands until evenly rounded. Place dough evenly on each sheet.

Bake cookies for 12 minutes. Remove cookies from oven and place one 1/2 inch piece of baking chocolate on the center top of each cookie, and then place half a marshmallow on top. Place cookies back in oven and bake for another 4 minutes.

Remove cookies from oven and let cookies cool for at least 5 minutes before transferring to a cooling rack. Garnish with chopped up baking chocolate if desired.