At this point, your oven should already be preheated to 350 degrees F.
In a large bowl, whisk together the flour, baking powder and salt. Set aside.
Using a double boiler, combine 4 ounces of the chocolate chips, unsweetened chocolate and butter in a heat-safe bowl. Stir together just until the chocolate has melted. Remove from heat and let the chocolate cool for 5-10 minutes.
In another large bowl, stir together the eggs, espresso powder, vanilla and sugar until just combined. Fold in the chocolate and then fold the dry ingredients into the wet. Stir until just moistened and then fold in the remaining chocolate chips.
Pour the batter into either 1 greased, large and deep (12 inch) cast iron pan or 4 individual cast iron pans (5 inch). Smooth out the top to even out the batter. Place them on a baking sheet and bake for 20-25 minutes or until just set on top. A toothpick will not come out clean.
Serve immediately with salty and sweet nuts and vanilla ice cream! These skillet brownies are best served straight out of the oven!