Meanwhile, combine butter, chocolate chips and cocoa in a medium bowl and mash together with a large spoon.
In a separate small bowl, combine eggs, vanilla and sugar. Mix to combine well.
Add egg mixture to chocolate and mix well, ensuring everything is well combined.
Add flour, baking powder and salt to the batter and stir until just combined.
When the cast iron skillet is hot, remove from heat and pour in batter. Cover with aluminum foil and place something heavy on top (i.e. cookie sheet, another pan inverted, lid, etc.).
Allow brownie to sit undisturbed for 20 minutes. Test doneness with a knife inserted until just barely clean. Cook over the open fire or on a stove for another 5 minutes if needed, but be careful to not let pan get too hot otherwise the bottom will burn.
Allow to cool slightly before serving directly from the skillet.
Be sure that you also have:
cast iron skillet
something heavy for insulation (i.e. lid, second skillet (any kind) or another pot – not required, but helpful)