Preheat the oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
Make the Cake Batter: In a large bowl, combine cake mix, mashed bananas, buttermilk, sour cream, oil, egg whites, and vanilla. Beat for 2 minutes until smooth.
Bake for 30–35 minutes or until a toothpick comes out clean. Let the cake cool completely.
Prepare Banana Cream Filling: Whisk pudding mix with milk until thick. In another bowl, whip cream and powdered sugar until stiff peaks form. Gently fold into pudding.
Spread filling over cooled cake. Refrigerate for at least 1 hour.
Make the Whipped Topping: Whip cream, sugar, and vanilla until stiff peaks form. Spread or pipe on top of the banana filling.
Garnish with banana slices and crushed wafers before serving. Enjoy chilled.