Preheat the oven to 425°. Brush four 6-ounce ramekins with melted butter. In a small bowl, whisk the cocoa powder with 1 tablespoon of the flour; dust the ramekins with the cocoa mixture, tapping out the excess. Transfer the ramekins to a sturdy baking sheet.
In a medium saucepan, melt 1 stick of butter with the chocolate over very low heat, stirring occasionally. Let cool slightly.
In a bowl, blend the peanut butter with the 1 tablespoon of confectioners’ sugar and the remaining 1 tablespoon of butter.
In a bowl, using an electric mixer, beat the granulated sugar with the eggs and salt at medium-high speed until thick and pale yellow, 3 minutes. Using a rubber spatula, fold in the melted chocolate until no streaks remain. Fold in the 1/4 cup of flour.
Spoon two-thirds of the batter into the prepared ramekins, then spoon the peanut butter mixture on top. Cover with the remaining chocolate batter. Bake in the center of the oven for 16 minutes, until the tops are cracked but the centers are still slightly jiggly. Transfer the ramekins to a rack and let cool for 5 to 8 minutes.
Run the tip of a small knife around each cake to loosen. Invert a small plate over each cake and, using pot holders, invert again. Carefully lift off the ramekins. Dust the warm cakes with confectioners’ sugar and serve immediately.