Preheat oven to 400F/200C. Generously butter 4-5 ramekins (depending on the ramekin size) and dust with cocoa powder, tapping the ramekins slightly to remove any excess.
Divide batter into prepared ramekins. Place a heaping teaspoon of peanut butter in the centre of each ramekin, on top of the batter, then slightly press it down using the back of the teaspoon into the batter until you can only see part of it (see photo above for a visual.). Bake for 11-13 minutes, until a thin crust has developed on top of the cake and the center jiggles slightly when gently shaken. If you want to loosen cakes from the ramekins, immediately run a thin knife around the edges and invert onto serving plates (if it’s too hot to handle you can let it cool for 5 minutes). Let the ramekins sit for a few seconds, then gently lift them off. Serve immediately. Dust with powdered sugar or cocoa powder or serve with whipped cream or ice cream if you like.