Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking pan with nonstick spray or a thin coating of butter. For extra insurance against sticking, you can line the bottom with parchment paper, but it’s usually not necessary.

In a medium bowl, combine the finely crushed vanilla wafers (2 cups), melted butter, and granulated sugar (if using). Mix until the crumbs are evenly coated and resemble wet sand. Press the mixture firmly into the bottom of the prepared pan using the bottom of a measuring cup or your hands. Bake for 8 minutes, then set aside to cool for about 5 minutes while you mix the cake batter.

In a large mixing bowl, prepare the yellow cake mix according to the package directions, using the eggs, oil, and water (or milk) as listed. For a richer, more bakery-style cake, you can swap the water for milk and add an extra egg yolk if desired. If you want extra banana flavor, mix in 1 mashed ripe banana and reduce the water or milk by 1/4 cup. Beat the batter on medium speed for about 2 minutes, scraping down the sides of the bowl so everything mixes evenly.

Pour the cake batter gently over the cooled vanilla wafer crust (or directly into the greased pan if you skipped the crust). Spread it into an even layer with a spatula. Bake at 350°F (175°C) for 25–30 minutes, or until the top is golden and a toothpick inserted in the center comes out clean or with a few moist crumbs. Place the pan on a cooling rack and let the cake cool for about 10–15 minutes. The cake should be warm but not piping hot when you poke the holes.

Using the handle of a wooden spoon, a thick straw, or a similar tool, poke holes all over the warm cake, spacing them about 1 inch apart. Poke almost all the way down to the bottom of the pan, especially if you used a vanilla wafer crust—you want the pudding to seep into the cake and reach the crust layer.

In a large bowl, whisk together the instant banana pudding mix and cold milk for about 2 minutes, until the mixture thickens but is still pourable. Let it sit for 1–2 minutes until it reaches the consistency of a thick custard—fluid enough to pour into the holes but not watery.

Slowly pour the pudding evenly over the surface of the warm, poked cake, aiming to fill the holes first. Use a spatula to gently push the pudding into the holes and spread any remaining pudding into an even layer over the top. You should see some pudding sinking down into the cake—that’s what creates the classic poke cake texture.

Arrange the sliced bananas in an even layer over the pudding. Overlap them slightly if needed—aim for a bit of banana in every bite. If your kitchen is warm, place the pan in the refrigerator for 15–20 minutes so the pudding can begin to set before you add the whipped topping.

Once the pudding has firmed up slightly, spread the thawed whipped topping in an even layer over the banana slices. Sprinkle the crushed vanilla wafers (for topping) generously over the whipped topping for added texture and that classic banana pudding look.

Cover the pan tightly with plastic wrap or a lid and refrigerate for at least 4 hours, or overnight for best flavor and texture. When ready to serve, slice the cake into squares and serve chilled. If desired, garnish each serving with extra banana slices and a pinch of crushed wafers just before serving to keep the bananas from browning.