In a bowl, whisk together the egg, sugar and vanilla extract until smooth. Add the cocoa powder, flour, baking powder and salt, and stir until a thick batter forms. Add the melted chocolate to the batter and stir to combine. Transfer the batter to a microwave-safe mug and then pop it in the microwave for 2 minutes.
Allow the cake to cool slightly and then insert the handle of a wooden spoon into the center of the cake to create a well. Fill the well with peanut butter cream. Microwave the marshmallow spread for 10 seconds or so, to soften it up. Top the cake with the marshmallow spread and dig in.