Preheat your oven to 350°F (or the temperature listed on your cake mix box). Grease a 9×13-inch baking pan with nonstick spray or butter. Prepare the yellow cake mix according to package directions (typically combining cake mix, eggs, oil, and water), then pour the batter into the prepared pan. Bake for 25–30 minutes, or as directed on the box, until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Place the baked cake on a wire rack and let it cool for 10–15 minutes. It should be warm but not hot. Using the handle of a wooden spoon, a chopstick, or a thick straw, poke holes all over the cake, spacing them about 1/2–1 inch apart. Gently twist as you poke so the holes are large enough for the banana pudding to soak into the cake.

In a large mixing bowl, whisk together the instant banana cream pudding mix and cold whole milk for about 2 minutes, until smooth and beginning to thicken. If you prefer a stronger banana flavor, whisk in the optional banana extract. Let the pudding stand for another 2–3 minutes; it should be pourable but not runny.

Pour the banana pudding mixture evenly over the warm, poked cake. Use an offset spatula or the back of a spoon to gently spread the pudding, applying light pressure so it seeps down into the holes. Some of the pudding will disappear into the cake as it soaks in—that’s what you want.

Cover the pan tightly with plastic wrap or foil and refrigerate for at least 2 hours, preferably 4 hours or overnight. This chilling time allows the pudding to fully set and soak into the cake, giving it a super moist, banana-pudding-like texture.

If using store-bought whipped topping, ensure it is fully thawed in the refrigerator. For homemade whipped cream, beat the heavy cream with powdered sugar and vanilla extract in a chilled bowl until medium-stiff peaks form. Use immediately and plan to serve the cake the same day, as homemade whipped cream is less stable than frozen topping.

Once the cake is fully chilled, spread the thawed whipped topping or homemade whipped cream evenly over the top in a thick, fluffy layer. Sprinkle the crushed vanilla wafers evenly over the whipped layer, leaving some larger pieces for crunch and some finely crushed for a “vanilla wafer crust” texture.

Right before serving, slice the bananas. For slower browning, very lightly toss the slices with a teaspoon of lemon juice. Scatter the banana slices over the top of the cake or add a few slices to each piece after cutting. Slice the cake into squares and serve chilled. Keep the pan refrigerated between servings.