Make the cake: For the cream cheese frosting, set aside 8 ounces of cream cheese and let it come to room temperature. To mix the ingredients, you have two options: either a stand mixer or an electric hand mixer in combination with a big bowl. Prior to beginning the baking process, position the oven rack to the central location and set the temperature to 350°F. Lubricate the interiors of two 9-inch circular cake pans with some oil and place rounds of parchment paper at their base.
Take a big mixing bowl and mix together 1 cup of full-fat sour cream, 1/2 cup of whole milk, 1/3 cup of vegetable oil, and 3 large eggs until well combined. Stir the ingredients until they form a smooth mixture. Then, add 1 box of cake mix and 1 box of banana cream pudding mix to the bowl and continue whisking until the mixture is uniform in consistency. Finally, divide the batter equally between the two prepared 9-inch cake pans.
To bake the cakes, set the oven timer for 20 to 25 minutes. Monitor the hue of the cakes, they should develop a light brown hue. To check if they are fully cooked, insert a toothpick in the center and if it comes out clean, the cakes are ready. Take the pans out of the oven and place them on a wire rack to fully cool. As the cakes cool, you can prepare the frosting. Make the frosting: To make the cream cheese mixture, add 1 1/4 cups of powdered sugar and 2 teaspoons of vanilla extract to a bowl of cream cheese. Using a whisk attachment, beat the ingredients on medium speed for 3 to 4 minutes until you achieve a smooth consistency. Then, lower the mixer speed and slowly add 3 cups of cold heavy cream. Keep whisking the mixture until it becomes thicker and smoother, which should take around 4 minutes. Subsequently, ramp up the mixer speed to medium and continue to beat until the frosting has doubled in volume and becomes light and fluffy. This should take 1 to 2 minutes. Set aside 1 cup of the frosting by transferring it to a small zip-top bag for later use.
Assemble the cake: Gently crush 3 1/2 cups of Nilla Wafers into coarse crumbs using a rolling pin or the bottom of a frying pan. To do this, place the wafers in a large zip-top bag, seal it, and apply pressure until the desired texture is achieved. Next, thinly slice 2 medium ripe (but not overripe) bananas and peel them.
Take the cakes out of the pans. Level off the rounded tops of each cake using a serrated knife. Next, arrange a cake layer with the cut side facing downwards on a serving plate or a cake stand. Spoon 1 cup of the frosting on top and evenly spread it to the edges using an offset spatula. Then, layer the sliced bananas over the frosting, followed by a uniform layer of 1 cup of crushed Nilla Wafers.
Put the final cake layer on top. Coat the top and sides of the cake with the remaining frosting, making sure to distribute it evenly. Then, take a handful of the remaining crushed Nilla Wafers and gently press them against the sides of the cake, starting from the bottom and working your way up, so they stick to the frosting. If any crumbs fall off, simply press them back into the frosting until the sides are fully coated.
Cut off one bottom corner of the frosting bag. Using the frosting, pipe small decorative mounds around the top edge of the cake. Insert a complete Nilla Wafer between each peak of frosting.