Preheat your oven to 425°F (220°C). Grease four ramekins with butter and dust with flour.
In a microwave-safe bowl, melt the butter and chocolate together in 30-second intervals, stirring after each, until smooth.
In a separate bowl, whisk together the flour, confectioners' sugar, eggs, egg yolks, vanilla extract, and salt until well combined.
Pour the melted chocolate mixture into the flour mixture and stir until smooth.
Divide half of the batter evenly among the prepared ramekins. Add a tablespoon of peanut butter to the center of each ramekin, then top with the remaining batter.
Bake for 12-14 minutes, or until the edges are set but the center is still soft.
Let the cakes cool in the ramekins for 1 minute, then run a knife around the edges to loosen and invert onto plates. Serve immediately.