Sift together cocoa powder, flour, baking powder and salt. Set aside.

In a medium saucepan, over medium heat, melt butter and the 6 oz of semisweet chocolate chips. Stir constantly until just melted. Remove from heat and let stand for 5 minutes to cool slightly.

In a large mixing bowl, combine the brown sugar, eggs and vanilla. Mix together on medium speed until combined.

Beat in the cooled chocolate mixture just until combined.

Add flour/cocoa mixture and blend on low speed just until combined.

Cover bowl with plastic wrap and refrigerate for 2 hours.

Preheat oven to 350 degrees. Line two cookie sheets with parchment paper.

Scoop and drop the cookie dough by the tablespoon, onto the prepared cookie sheets.Space them at least 2 inches apart.

Bake cookies 350º F for 12 minutes.

Remove cookies from oven and working quickly so cookies don't cool too much, place one half piece of marshmallow on top of each cookie.

Place the cookies back in the oven and bake an additional 4 minutes. If you want the tops browned a bit more, turn oven onto broil and bake for a few seconds. keep a close eye so they don't burn!

Let cookies cool at least 5 minutes before transferring to a cooling rack.