01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
02 - In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
03 - In a large bowl, cream the butter and sugar until light and fluffy. Beat in the eggs one at a time. Add the mashed bananas and vanilla extract, mixing until combined.
04 - Add the dry ingredients to the wet mixture in three additions, alternating with the buttermilk, beginning and ending with the dry mix. Mix just until combined.
05 - Divide the batter evenly between the prepared pans. Bake for 25–30 minutes, until a toothpick inserted in the center comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
06 - While the cakes cool, whisk the pudding mix and cold milk in a bowl until thickened, about 2 minutes. In a separate bowl, whip the heavy cream with powdered sugar and vanilla to soft peaks. Gently fold the whipped cream into the pudding.
07 - Place one cake layer on a serving plate. Spread a generous layer of banana pudding filling over the top. Arrange banana slices 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 wafers, and whipped cream if using.
09 - Refrigerate for at least 2 hours before serving to allow the flavors to meld and the cake to set properly.