1. Preheat oven to 350 degrees. Butter an 8-inch seasoned cast-iron skillet; set aside. Place chocolate and butter in a medium heatproof bowl. Set over a pan of simmering water; stir until almost melted. Remove from heat; let cool, stirring. Set aside.

2. Into a medium bowl, sift together the flour, cocoa, baking powder, and salt. Sift again, and set aside. In the bowl of an electric mixer fitted with the paddle attachment, beat the eggs and vanilla until thick and pale, about 6 minutes. Beat in the sugar until fluffy. Stir in the chocolate mixture. Fold in the dry ingredients until just combined.

3. Pour the batter into prepared skillet. Bake until spoon bread is set but still soft in the center, about 40 minutes (cook less for a more molten center). Let cool 10 minutes; serve warm in the skillet.