In a small mixing bowl, whisk together peanut butter, egg and oil.
Stir in milk, vanilla and brown sugar. Then, flour.
Fold in marshmallow fluff.
Divide batter between 2 mugs evenly.
Top with another tablespoon of marshmallow fluff.
Microwave separately for 2 minutes or until baked through (depending on microwave, you may need just another 20-30 seconds)
For best taste, eat immediately