Bake cake in a 9x13-inch pan according to directions on the box. Remove the cake from the oven and poke holes all over the cake with a wooden spoon handle.
Whisk together pudding and milk. Pour over cake, making sure it gets down in the holes. Put a layer of Nilla wafers on top of the cake. Put cake in the refrigerator and allow cake to cool completely.
Once cake has cooled and pudding is set, slice bananas and layer on top of the Nilla wafers.
Whip together heavy cream, powdered sugar and vanilla extract. (I chill my mixing bowl in the freezer for 5-10 minutes before adding the cream and sugar). Whip at high speed for 1-2 minutes. Spread over cake and top with crushed Nilla wafers.
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