For the filling: Mix together the heavy cream, peanut butter, powdered sugar and vanilla extract and set aside.
For the cake: Add the chocolate chips and butter to a small bowl and microwave for 20 to 30 seconds to melt. Stir and set aside.
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.
Recipe courtesy of Kelsey Nixon