In a medium saucepan, over low heat, melt the butter and the 12 ounces of semisweet chocolate chips. Stir constantly until just melted. Remove from heat and let stand for 5 minutes to slightly cool.
Beat sugar, eggs, and vanilla then add chocolate mixture and mix until combined.
Add the cocoa powder, flour, baking powder, and salt. Mix on low speed just until combined. Cover bowl with plastic wrap and refrigerate for 2 hours.
Preheat oven to 325F. Line two cookie sheets with parchment paper. Scoop and drop the cookie dough by the tablespoon, onto the prepared cookie sheets. You should fit 12 cookies per sheet, and make sure to space them at least 2 inches apart.
Bake cookies for 12 minutes.
Remove cookies from oven and place one piece of chocolate on the center top of each cookie.
Cut marshmallows in half, then top each chocolate with one-half marshmallow. Place cookies back in oven and bake another 4 minutes so the marshmallows melt.
Place cookies back in oven and bake another 4 minutes so the marshmallows melt. Sprinkle with grated semisweet chocolate or lightly dust with cocoa powder and allow to cool or enjoy warm!