Preheat the oven to 350°F and lightly spray a 13 x 9 inch baking dish with cooking spray. Set aside. In a medium size mixing bowl using an electric mixer, beat together the cake mix, ½ cup melted butter and 1 egg. Beat until fully moistened then press onto the bottom of the baking dish.
In the same bowl, cream together the cream cheese, ½ cup melted butter, powdered sugar, banana cream pudding mix and vanilla. Beat until combined.
Add the remaining 3 eggs one at a time, beating well after each. After all of the eggs have been added increase the speed of the mixer and beat for 1-2 minutes until pale yellow and fluffy. Mix in the mashed banana by hand. Spread onto the crust.
Place into the oven and bake for 40-45 minutes or until set. Cake should still "jiggle" when gently shaken. Don't over bake. Cool completely on a baking rack then chill thoroughly.
To serve, divide into 20 pieces lightly dust with powdered sugar and garnish each piece with whipped cream and a vanilla wafer.
Store leftovers chilled.