Preheat oven to 350° F.: Close your oven door and listen for the faint hum as it warms, a sign it is reaching temperature. The consistent oven heat encourages even browning on the crust edges, so I always let the oven fully preheat before placing the pan inside. If the oven is cooler than indicated, the crust will take longer to firm up and may end up denser. A common mistake is rushing this step, which can leave you with an underbaked base, so allow those minutes to pass for predictable results.
Spray a 13×9-inch cake pan with cooking spray. Line with parchment paper, leaving an overhang over the sides. This will help you lift the bars out of the pan later.: The parchment overhang acts like handles when you extract the set bars, so make sure it extends at least an inch over the long sides. The spritz of cooking spray keeps the parchment from sliding around while you press in the crust. If you skip the parchment, you risk the crust sticking and breaking when you try to remove it, so take the extra minute to prepare the pan properly.
In a food processor, blend together flour, butter, powdered sugar, and salt until the mixture resembles coarse sand. This will not look like dough – that’s ok! Press firmly into the bottom of the prepared pan. Bake until the edges are golden, about 20 minutes. Let cool completely.: As the blades whirl, you’ll see the texture change from separate granules to a cohesive, sandy mass, with small visible bits of cold butter . Press the mixture firmly, using the bottom of a measuring cup for an even surface, because a compacted crust will hold the filling without crumbling. When the edges develop a warm golden tone and you catch a nutty, baked aroma, it’s ready. A frequent error is underpressing the crust, which causes it to flake apart when sliced, so apply firm, even pressure for a sturdy base.
In a large bowl, whisk together the sweetened condensed milk and whole milk to combine. Whisk in the pudding mix. Whisk for two minutes.: You want the mixture smooth and slightly thickened, with no visible granules of pudding. Whisking for the full two minutes activates the instant pudding’s thickening properties, which yields a silkier set. Smell a clean vanilla scent and note the mixture gaining body as air is incorporated. If you underwhisk, the pudding may not set uniformly and you could end up with a runny layer, so keep that wrist moving.
Spread half of the pudding mixture over the cooled crust. Top evenly with the sliced bananas. Spread the remaining pudding over the top. Cover and refrigerate for at least 2-3 hours.: When you spread the first pudding layer, use gentle, even strokes to avoid disturbing the crust beneath. Lay the bananas in slightly overlapping rows so each square gets fruit. The second pudding layer seals the fruit and prevents browning, while chilling allows the filling to firm into a sliceable consistency. A misstep is adding the bananas too early before the first layer sets, which can cause them to sink and create an uneven filling.
Spread the thawed whipped topping over the top of the filling.: Gently fold the topping if necessary to restore fluffiness, then spread with a flexible spatula in smooth strokes to achieve an even white cap. The whipped topping should feel airy and cold against the skin, and it provides a pleasing contrast to the denser pudding beneath. If you work the topping too vigorously, it may become loose and start to weep, so be tender with it.
Use the parchment paper to lift the bars out of the pan. Cut into slices and serve.: Pull up on the parchment overhang slowly so the bars rise intact, then place the block on a cutting board. Use a sharp knife dipped briefly in warm water and wiped dry between cuts for clean edges. You should hear a soft give as the knife passes through the chilled layers, and the cross section will reveal distinct crust, pudding, and topping. Cutting with a dull blade leads to ragged slices and smudged layers, so keep the knife sharp and clean.
If desired, garnish with additional slices of banana or vanilla wafer crumbs just before serving.: A few thin bananas on top brighten the presentation, and vanilla wafer crumbs add a playful crunch. Arrange garnishes sparingly so they do not overwhelm the creamy surface; you want contrast, not clutter. Avoid making the garnish too early because moisture from the filling can make it soggy, so add it immediately before serving for the freshest look.
