01 - Preheat the oven to 350°F. Grease and flour two 9-inch round cake pans and line the bottoms with parchment paper.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until evenly distributed.
03 - In a large bowl, beat the softened butter and granulated sugar together using an electric mixer until the mixture is light and fluffy, approximately 3 to 4 minutes.
04 - Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract until fully combined.
05 - Blend in the mashed bananas until evenly distributed. Alternately add the flour mixture and buttermilk to the wet ingredients, beginning and ending with the flour mixture. Mix until just combined, being careful not to overmix.
06 - Divide the batter evenly between the two prepared cake pans. Bake for 30 to 35 minutes, or until a toothpick inserted into the center of each layer comes out clean.
07 - Allow the cakes to cool in their pans for 10 minutes, then carefully turn them out onto a wire rack to cool completely.
08 - Whisk the instant vanilla pudding mix and cold whole milk together until smooth and thickened, about 2 minutes. Gently fold in the whipped heavy cream until uniformly combined.
09 - Place one cooled cake layer on a serving platter. Spread half of the pudding filling evenly over the top. Arrange the sliced bananas over the filling and sprinkle with a layer of crushed vanilla wafer cookies.
10 - Carefully place the second cake layer on top. Spread the remaining pudding filling over it and add additional banana slices and crushed vanilla wafers.
11 - Beat the heavy whipping cream with powdered sugar and vanilla extract until stiff peaks form. Spread or pipe the whipped cream over the top and sides of the assembled cake.
12 - Refrigerate the assembled cake for at least 2 hours to allow the layers to set properly. Garnish with additional banana slices and wafer crumbs before serving if desired.