Preheat the oven to 350 degrees. Spray a 9 x 13 baking dish with cooking spray and set aside.

In a medium bowl, combine the flour, sugar, baking powder, and salt. Set aside.

In another large bowl, whisk together the egg, buttermilk, sour cream, oil, and vanilla extract until thoroughly combined.

Pour the wet ingredients into the dry ingredients and stir with a wooden spoon until just combined.

Transfer the cake batter into prepared pan.

Bake cake for 30 minutes, or until a toothpick comes out clean when inserted into the center of the cake.

Remove from the oven and let cake cool completely on a wire rack.

Once cake has cooled, make the pudding. In a large bowl, using an electric mixer, mix the milk and the pudding mix for 2 minutes. Let the pudding rest for 3 minutes, or until pudding has thickened.

Poke holes over the top of the cooled cake with the thick end of a wooden spoon.

Spread chilled pudding evenly over top of the cake and arrange sliced bananas over top of the pudding.

To make the whipped topping, combine the heavy cream, powdered sugar, and vanilla extract in a large, non reactive mixing bowl. Mix on medium speed with a hand mixer until the cream has become stable.

Evenly spread the whipped cream over the top of the banana cake. Cover the cake tightly with plastic wrap and refrigerate 6 hours or overnight.