Preheat oven to 350°F. Spray a 9x13 baking pan with nonstick cooking spray.
In a large bowl, combine yellow cake mix, water, vegetable oil, and eggs. Beat on medium speed for 2 minutes until smooth and well combined.
Pour batter into prepared pan and spread evenly. Bake for 25-30 minutes until a toothpick inserted in center comes out clean.
Remove cake from oven and let cool for exactly 10 minutes on a wire rack.
Use the handle of a wooden spoon to poke holes all over the cake, spacing them about 1 inch apart and going 3/4 of the way down.
In a medium bowl, whisk together banana pudding mix and 3 cups cold milk for 2 minutes until mixture begins to thicken but is still pourable.
Slowly pour pudding mixture over the warm cake, using a spatula to gently spread it and help it seep into the holes.
Refrigerate cake for at least 2 hours or overnight until completely set and chilled.
Before serving, spread whipped topping over the top of the cake. Sprinkle with crushed vanilla wafers and arrange sliced bananas on top.
Cut into squares and serve immediately. Keep refrigerated when not serving.