Steps:

• Make the cake: Place the 8 ounces cream cheese for the frosting in the bowl of a stand mixer and let it sit out at room temperature while you make the cake. (Alternatively, use an electric hand mixer and large bowl.) Arrange a rack in the middle of the oven and heat the oven to 350°F. Coat 2 (9-inch) round cake pans with cooking spray, then line the bottoms with rounds of parchment paper.
• Place 1 cup full-fat sour cream, 1/2 cup whole milk, 1/3 cup vegetable oil, and 3 large eggs in a large bowl and whisk until smooth. Add 1 box cake mix and 1 box banana cream pudding mix and whisk until smooth. Evenly divide the batter between the prepared baking pans.
• Bake until the cakes are lightly browned and a toothpick inserted into the center comes out clean, 20 to 25 minutes. Place the pans on a wire rack and cool completely. Meanwhile, make the frosting.
• Make the frosting: Add 1 1/4 cups powdered sugar and 2 teaspoons vanilla extract to the bowl of cream cheese. Beat with the whisk attachment on medium speed until smooth, 3 to 4 minutes. Reduce the mixer speed to low and gradually pour in 3 cups cold heavy cream. Continue beating until the mixture thickens and is smooth, 4 minutes. Increase the speed to medium and beat until the frosting is light, fluffy, and doubled in volume, 1 to 2 minutes. Transfer 1 cup of the frosting to a small zip-top bag and set aside.
• Assemble the cake: Place 3 1/2 cups of the Nilla Wafers in a large zip-top bag. Seal the bag and gently crush the wafers with a rolling pin or the bottom of the frying pan into coarse crumbs. Peel and thinly slice 2 medium ripe (but not spotty) bananas.
• Remove the cakes from the pans. Using a serrated knife, trim the rounded tops off of each cake. Place one cake layer, cut-side down, onto a serving platter or cake stand. Dollop with 1 cup of the frosting and smooth into a thin layer from edge to edge with an offset spatula. Top with the sliced bananas, then 1 cup of the crushed Nilla Wafers in an even layer.
• Top with the remaining cake layer. Frost the top and sides of the cake with the remaining frosting. Cover the sides of the cake with the remaining crushed Nilla Wafers by taking a handful of the crushed cookies and gently pressing them against the side of the cake (start at the bottom and work your way up) so they stick to the frosting. Any pieces that don’t stick and fall off can be pressed back into the frosting until the sides are completely covered.
• Snip off one bottom corner of the bag of frosting. Pipe small decorative mounds of the frosting around the top of the cake. Insert a whole Nilla Wafer between each mound of frosting.