Preheat your oven to the temperature listed on the yellow cake mix box. Prepare the cake batter according to package directions and pour into a 9x13 inch baking dish. Bake for the time specified on the box until a toothpick inserted in the center comes out clean. This creates the foundation for absorbing the pudding mixture.

Allow the baked cake to cool for about 5 minutes—this slight warmth helps the pudding soak in better. Using the handle of a wooden spoon, poke holes throughout the entire cake about 1 inch apart, working methodically across the surface. While the cake cools, whisk together the milk and banana pudding mix in a bowl until smooth and well combined, about 1-2 minutes. I like to make sure there are no lumps in the pudding mixture before pouring it over the cake, so I'll strain it through a fine sieve if needed.

Slowly pour the pudding mixture over the entire surface of the poked cake, allowing it to seep into all the holes and distribute evenly. Cover the cake with plastic wrap and refrigerate for at least 1 hour—this allows the pudding to fully set and the cake to absorb the flavors completely. The longer you can chill it, the better the pudding integrates with the cake.

Remove the chilled cake from the refrigerator. Spread the thawed whipped topping evenly over the entire surface using an offset spatula or the back of a spoon, creating an even layer. Arrange the banana slices in a decorative pattern over the whipped topping, pressing them gently so they stay in place. I find it helpful to slice the bananas just before this step to keep them fresh and prevent browning.

Crush or crumble the nilla wafers into small pieces and sprinkle them over the top of the cake just before serving, ensuring they stay crispy. Slice and serve immediately—the contrast between the creamy pudding, fluffy topping, fresh bananas, and crunchy cookies is best enjoyed fresh.