Step 1

Prepare the Custard Filling: Whisk together sugar, cornstarch, and salt in a small, heavy saucepan. Whisk together egg yolks and half-and-half in a large glass measuring cup or bowl; gradually whisk into sugar mixture. Cook over medium, whisking constantly, until mixture starts to bubble, 3 to 4 minutes. Continue cooking, whisking constantly, 1 minute. Remove from heat. Add butter and vanilla; whisk until completely incorporated. Transfer mixture to a medium bowl. Place plastic wrap directly on surface of warm mixture to prevent film from forming. Let stand at room temperature 30 minutes. Transfer to refrigerator, and chill until cold, at least 4 hours or up to 8 hours.

Step 2

Prepare the Cake Layers: reheat oven to 350°F. Coat 3 (8-inch) round cake pans with baking spray; set aside. Beat cake mix, eggs, milk, melted butter, and vanilla with a stand mixer fitted with a paddle attachment on low speed until incorporated, about 30 seconds. Increase mixer speed to medium; beat 2 minutes. Divide batter evenly among prepared pans.

Step 3

Bake Cake Layers on middle rack in preheated oven until lightly golden and a wooden pick inserted in center comes out clean, 16 to 18 minutes. Transfer pans to a wire rack; let cool 10 minutes. Remove Cake Layers from pans, and let cool completely on wire rack, about 1 hour.

Step 4

Return to the Custard Filling: Beat whipping cream with an electric mixer on high speed until stiff peaks form, 1 to 3 minutes. Remove Custard Filling from refrigerator; gently fold whipped cream into chilled Custard Filling. Place in refrigerator until ready to use.

Step 5

Prepare the Whipped Cream Frosting: Beat whipping cream and vanilla with an electric mixer on medium-high speed until foamy, about 1 minute. Gradually add sugar, beating until stiff peaks form, about 2 minutes. (Do not overbeat.)

Step 6

Assemble the cake: Using a serrated knife, horizontally slice a very thin layer off the top of each Cake Layer to remove any domes, and discard. Place 1 Cake Layer, cut side up, on a platter; cover Cake Layer with half of the sliced bananas, covering top of layer. Spread half of the Custard Filling (about 1 cup) over top of bananas. Top with second Cake Layer, cut side up; repeat layering with remaining bananas and Custard Filling. Top with remaining Cake Layer, cut side down. Spread top and sides of assembled cake with Whipped Cream Frosting.

Step 7

Place wafers in a ziplock plastic bag; seal bag, and coarsely crush wafers into small pieces using your hands. Gently press some of the crushed wafers around bottom of cake (about 2 to 3 inches up the sides). Garnish top of cake with whole wafers and remaining crushed wafers.