Preheat your oven to 350 degrees F. This step is crucial as it allows your cake to bake evenly. While waiting, grease only the bottom of a Pyrex 9x13 inch deep baking dish. This will help the cake release easily after baking.
In a large mixing bowl, pour the yellow cake mix. Then, add the mashed bananas, eggs, vegetable oil, and water. Whisk the mixture until it’s mostly smooth. A few small chunks of banana left are fine, as they add texture to the cake.
Pour the cake batter into the prepared Pyrex dish. Make sure to spread it evenly in the pan so it bakes uniformly.
Bake undisturbed for 30 minutes. To check if it’s done, insert a toothpick into the center of the cake. If it comes out clean, you know the cake is perfect!
Allow the cake to cool completely before layering the top. This step is important; a hot cake will melt the pudding, ruining the layers.
While the cake is cooling, prepare the pudding layer. In the bowl of an electric stand mixer, pour the instant pudding mix. Add whole milk and beat for about 2 minutes until it thickens. Using slightly less milk than the box suggests creates a thicker pudding, which is perfect for layering.
Once your cake has cooled, spread the pudding evenly over the top. Then, cover the pudding layer with the banana slices, making sure they’re nicely distributed.
Next, wipe out the stand mixer bowl and add in the heavy cream and granulated sugar. Using the whip attachment, whip the cream on high until it forms firm peaks. You want it to be thick enough to hold its shape.
Carefully spread the whipped cream over the bananas, ensuring they are completely covered. This fluffy layer is what makes the cake so indulgent.
Finally, sprinkle the top with the crushed vanilla wafers. This adds a delightful crunch and finishing touch. Cover the dish and refrigerate until you’re ready to serve, allowing the flavors to meld together beautifully.