Preheat the oven to 220C/200C Fan/425F/Gas 7. Grease four 4-oz ramekins, and place them on a baking tray.
Add the chocolate and butter to a large microwave-safe bowl and heat in 20 second intervals, stirring after each one, until melted and smooth.
Add the eggs, egg yolks, and vanilla, and whisk until combined. Add the icing sugar and flour, and whisk until smooth and combined.
Divide the mixture between the prepared ramekins.
Add 1 heaped teaspoon of peanut butter to the centre of each cake, pressing down gently to almost submerge.
Bake for 12 - 14 minutes, or until the sides of the cakes are firm, and the centres are still soft and gooey.
Carefully run a knife around the edges of each cake, then turn them out onto serving plates. Serve immediately.
Video
Notes
Make ahead of time: Simply prepare the pudding and divide between the ramekins. Cover tightly with clingfilm and keep refrigerated for up to 24 hours. Bake when ready to serve.