Preheat your oven to 350°F (or as per your cake mix instructions). Grease and flour a 9x13-inch baking dish. Mix the yellow cake mix, eggs, oil, and water according to the package directions. Pour the batter into your prepared dish and bake until a toothpick inserted in the center comes out clean, usually 25-30 minutes.
Remove the cake from the oven and let it cool for about 10 minutes. Using the handle of a wooden spoon or a similar tool, gently poke holes over the entire surface of the cake. The more holes, the more pudding soaks in!
In a large bowl, whisk together instant banana pudding mixes and cold milk until smooth and starting to thicken. Pour the pudding evenly over the still-warm cake, making sure it seeps into all the holes.
Cover the cake and refrigerate for at least 2 hours, or until the pudding is set and the cake is well chilled.
Once chilled, spread the thawed whipped topping evenly over the entire cake. Sprinkle the crushed vanilla wafers over the top right before serving for that classic crunch.
Notes
For maximum flavor, use ripe bananas as an optional garnish or sliced in between the pudding and whipped topping. Can’t find banana pudding? Substitute with vanilla pudding and add a few drops of banana extract for that signature taste.