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 wet mixture until evenly incorporated throughout the batter.
06 - Alternately add the flour mixture and buttermilk to the wet ingredients, beginning and ending with the flour. Mix until just combined, being careful not to overmix.
07 - Divide the batter evenly between the two prepared pans. Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
08 - Allow the cakes to cool in their pans for 10 minutes, then turn them out onto a wire rack to cool completely.
09 - 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.
10 - Place one cooled cake layer on a serving platter. Spread half of the pudding filling evenly over the top. Arrange sliced bananas and a layer of crushed vanilla wafers on top of the filling.
11 - Carefully place the second cake layer on top. Spread the remaining pudding filling over it, then add more sliced bananas and crushed vanilla wafers.
12 - 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.
13 - Refrigerate the cake for at least 2 hours to allow the filling to set properly. Garnish with additional banana slices and wafer crumbs before serving if desired.