
Preheat the oven to 350°F. Line the bottoms of two greased 9-inch round baking pans with parchment and grease the paper. Combine the yellow cake mix, eggs, oil, milk and vanilla extract, and beat on low speed until moistened, about 30 seconds. Beat on medium speed for two minutes. Stir in the mashed banana.
Spread 1/2 cup of the cake batter into each prepared pan. Arrange 2 cups of vanilla wafers on top of the batter, flat side up (1 cup per pan). Gently spread the remaining batter over the wafers.
Bake the cake for 15 to 20 minutes until a toothpick inserted in the center comes out clean. Cool the cakes in the pans for 10 minutes before removing them from the pans to wire racks to cool completely.
Whisk together the milk and pudding mix for two minutes. Let the mixture stand for two minutes until soft-set.
In a chilled large bowl and using chilled beaters, beat the heavy cream until it begins to thicken. Add the confectioners' sugar and beat until soft peaks form.
Place one cake layer on a serving plate and spread the top with the pudding mixture. Top with the second cake layer. Spread the sweetened whipped cream over the top and sides of the cake. Crush 1 cup of vanilla wafers and gently press the crumbs onto the sides of the cake. Garnish the cake with the remaining whole vanilla wafers and sliced bananas if desired.