Preheat the oven to 350 degrees F.
Combine the cake mix, egg, and butter together and mix well. Pat into a lightly greased glass 13 by 9-inch baking pan. Prepare the filling.
Beat the cream cheese until smooth. Add the eggs and vanilla. Add the peanut butter; beat. Add the banana and butter and mix well. Add the powdered sugar and mix well. Spread over the cake mixture. Bake for 45 to 50 minutes. You want the center to be a little gooey, so do not over bake. Top each cake slice with a mint leaf and dollop of whipped cream