A well-seasoned cast iron skillet acts similar to a nonstick skillet, making just a light brush of butter all that’s needed to prevent sticking in this dish. This cocoa powder-based brownie will end up more chewy than fudgy, with crisp edges. Slow cooking over indirect heat will make them perfectly moist.
Preheat a grill to medium and prepare for indirect cooking: On a gas grill, turn off half the burners; on a charcoal grill, bank the coals to one side.
Lightly butter the bottom and sides of a well-seasoned 10-inch cast-iron skillet. Melt the butter in a metal bowl over indirect heat on the grill; let cool slightly. Whisk in the granulated sugar, brown sugar, eggs and vanilla until smooth. Add the flour, cocoa powder, salt and baking powder and whisk until well combined. Stir in the chocolate chips. Spread the batter in the skillet.
Once the grill registers 325˚ F to 350˚ F, set the skillet on the cooler side of the grill (indirect heat), cover the grill and cook until the edges of the brownie are set and a toothpick inserted into the center comes out clean, 40 to 50 minutes. Remove from the grill and let the brownie cool in the skillet at least 20 minutes.
Meanwhile, beat the heavy cream to soft peaks in a medium bowl with a mixer on medium-high speed. Add the marshmallow cream and continue beating to firm peaks. Serve the brownie with the whipped cream.
Make your grill act like an oven: Place the brownie over indirect heat and cover the grill as it bakes.