To make the cake, preheat the oven to 350°F/180°C, and spray the bottom of a 9x13-inch baking dish with baking spray. Set aside.
In a bowl whisk together milk with eggs, melted butter, and vanilla extract. Add cake mix powder, and whisk until just combined. Pour the mixture into the prepared pan, and bake until a toothpick comes out clean out of the center of the cake about 25 minutes. Remove from the oven, and allow to cool completely.
Use the end of a wooden spoon, and poke holes in the cake about 1 inch apart.
To make the banana pudding: In a separate bowl, combine the milk with instant pudding powder and whisk for 2 minutes or until slightly thickened but still pourable.
Pour the banana pudding into the holes of the cake, then chill in the fridge for about one hour.
Layer the sliced bananas all over the cake in a single layer.
Make the whipped cream topping: In a bowl of a stand mixer or in a bowl and using a hand mixer, combine heavy whipping cream with powdered sugar, and vanilla, and whip until soft peaks form.
Spread the topping on the bananas, garnish with more banana slices and crushed vanilla wafers. Slice and serve.