Make the cake as follows: Allow the 8 ounces of cream cheese frosting to come to room temperature in the bowl of a stand mixer while you make the cake. (Alternatively, an electric hand mixer and a large mixing bowl can be used.) Preheat the oven to 350°F and place a rack in the center. Coat two (9-inch) round cake pans with cooking spray and line the bottoms with parchment paper rounds.
In a large mixing bowl, whisk together 1 cup full-fat sour cream, 1/2 cup whole milk, 1/3 cup vegetable oil, and 3 large eggs until smooth. Whisk in 1 box cake mix and 1 box banana cream pudding mix until smooth. Divide the batter evenly between the prepared baking pans.
Bake for 20 to 25 minutes, or until the cakes are lightly browned and a toothpick inserted into the center comes out clean. Cool completely in the pans on a wire rack. In the meantime, prepare the frosting.
To make the icing: In a mixing bowl, combine 1 1/4 cups powdered sugar and 2 teaspoons vanilla extract. 3 to 4 minutes on medium speed with the whisk attachment, until smooth. Reduce the mixer speed to low and slowly drizzle in 3 cups of cold heavy cream. Continue beating for 4 minutes, or until the mixture thickens and smoothes. Increase the speed to medium and beat for 1 to 2 minutes, or until the frosting is light, fluffy, and doubled in volume. Set aside 1 cup of the frosting in a small zip-top bag.
Assemble the cake as follows: In a large zip-top bag, place 3 1/2 cups Nilla Wafers. Seal the bag and gently crush the wafers into coarse crumbs with a rolling pin or the bottom of a frying pan. 2 medium-ripe (but not spotty) bananas, peeled and thinly sliced
Take the cakes out of the pans. Trim the rounded tops of each cake with a serrated knife. Place one cake layer on a serving platter or cake stand, cut-side down. 1 cup frosting, dollop on top and smooth into a thin layer from edge to edge with an offset spatula. Top with sliced bananas, followed by 1 cup crushed Nilla Wafers in an even layer.
Finish with the last cake layer. With the remaining frosting, frost the top and sides of the cake. Cover the sides of the cake with the remaining crushed Nilla Wafers by gently pressing a handful of the crushed cookies against the side of the cake (starting at the bottom and working your way up) so they stick to the frosting. Any pieces that do not stick and fall off can be pressed back into the frosting until completely covered.
Snip one bottom corner of the frosting bag. Small decorative mounds of frosting should be piped around the top of the cake. Place an entire Nilla Wafer between each mound of frosting.
