01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
02 - In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, use an electric mixer to beat the softened butter and granulated sugar on medium speed until light and fluffy, about 3 minutes. Add the eggs one at a time, beating well after each addition. Mix in the mashed bananas and vanilla extract until thoroughly combined.
04 - Add the dry ingredient mixture to the wet ingredients in three additions, alternating with the buttermilk. Begin and end with the dry mixture. Mix just until combined—do not overmix.
05 - Divide the batter evenly between the two prepared cake pans. Bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
06 - While the cakes cool, whisk the instant vanilla pudding mix with cold whole milk in a bowl until thickened, about 2 minutes. In a separate bowl, whip the heavy cream with powdered sugar and vanilla extract to soft peaks. Gently fold the whipped cream into the pudding mixture until smooth and uniform.
07 - Place one cooled cake layer on a serving plate. Spread a generous, even layer of the banana pudding filling over the top. Arrange sliced bananas over the filling and sprinkle with a layer of crushed vanilla wafers.
08 - Place the second cake layer on top. Spread the remaining pudding filling over the top and sides if desired. Garnish with additional banana slices, crushed vanilla wafers, and whipped cream if using.
09 - Refrigerate the assembled cake for at least 2 hours before serving to allow the flavors to meld and the layers to set properly.