In a medium bowl, mix the flour, cocoa powder, castor sugar and baking powder.

Gently add the milk and oil and whisk well until all the ingredients are combined and the batter had no lumps.

Pour the batter into a microwave-safe mug. Don't fill the mug right up to the brim as the cake shall rise and you would not want it to spill over !!! (I typically fill a mug to about two-thirds, no more !! Divide the batter into two mugs if required.)

Now gently push the spoonful of peanut butter into the middle of the mug.

Microwave for 60 seconds (900 watts, microwave mode). Allow to rest for a further 30 seconds. Serve warm.