01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans, then 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 with an electric mixer until 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 - Fold the mashed bananas into the batter. Alternately add the flour mixture and buttermilk in three additions, beginning and ending with the flour mixture. Mix until just combined to avoid overworking the batter.
06 - Divide the batter evenly between the two prepared 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 invert onto a wire rack to cool completely.
08 - Whisk the instant vanilla pudding mix with cold whole milk until smooth and thickened, about 2 minutes. Gently fold in the whipped heavy cream until no streaks remain.
09 - Place the first cake layer on a serving platter. Spread half of the pudding filling evenly over the top. Arrange sliced bananas over the filling and sprinkle with a layer of crushed vanilla wafers.
10 - Carefully place the second cake layer on top. Spread the remaining pudding filling over it and finish with 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 cake for at least 2 hours to allow the layers to set. Garnish with extra banana slices and wafer crumbs before serving if desired.