Preheat oven to 350 degrees. Butter an 8-inch, well-seasoned cast iron skillet, set aside.
Place chocolate and butter in a medium heat-proof bowl. Set over a pan of boiling water and whisk together until almost blended. Remove from heat and let cool. Set aside.
Sift togetther the flour, cocoa, baking powder, and salt. Sift again and set aside.
In another bowl, beat the eggs and vanilla until thick and pale, about 6 minutes. Beat in the sugar until fluffy. Stir in the chocolate mixture. Fold the dry ingredients gently until just combined.
Pour the batter into prepared skillet. Bake until spoon bread is set but still soft in center, about 40 minutes. (cook less for a more molten center.) Let cool 10 minutes serve warm. Dust with confectioner's sugar to garnish, if desired.