Make cake: Preheat oven to 350°. Line two round 9″ cake pans with parchment paper and grease with cooking spray. Prepare cake mix according to package instructions. Divide batter between cake pans and bake until a toothpick inserted into the center comes out clean, 25 to 30 minutes. Let cakes cool for 10 minutes in pans then invert onto wire racks to cool completely.
Make pudding: In a large bowl, combine pudding mix and milk and stir until thickened, about 5 minutes. Fold in Cool Whip.
Make buttercream frosting: In a large bowl using a hand mixer, combine butter, 3 cups powdered sugar, vanilla, and salt. Beat until light and fluffy. Add remaining sugar and heavy cream and beat until smooth.
Assemble cake: Using a serrated knife, cut off the rounded top of each cake to level. Top one cake with a layer of pudding then top with most of the banana slices (reserve some for garnish). Place second cake on top and frost entire cake with buttercream, reserving about 1 cup to pipe on top. Press crushed Nilla wafers around the sides of the cake.
Transfer remaining buttercream to a piping bag fitted with a large star tip and pipe large swirls around the top of the cake. Decorate with Nilla wafers and more banana slices.