Preheat the oven to 350°F. Grease the bottom and sides of a 10-inch cast-iron skillet generously with butter.
In a medium-sized saucepan, melt the butter over medium heat. Add the sugar and stir to combine. Cook, stirring frequently, until the mixture is just starting to bubble at the edges, but not boiling. Remove from heat.
In a large mixing bowl, whisk together eggs, cocoa, baking powder, salt, and vanilla till smooth. Add the hot butter/sugar mixture, stirring until smooth. Stir in the flour just until blended.
Let batter cool 15 minutes, then stir in half of the chocolate morsels. Spread the batter into the prepared skillet. Sprinkle remaining chocolate morsels over the top.
Bake for 30 - 35 minutes, until a toothpick inserted into the center comes out with a few moist crumbs clinging to it. Don’t over bake. Remove the skillet from the oven and cool completely on a rack.