Preheat the oven to 350°F. Spray a 9x13-inch baking pan with nonstick spray and set aside.
Prepare the cake mix according to box instructions. Pour the batter into the prepared pan and bake according to the package directions. Allow to cool for 10 minutes.
15.25 ounces box yellow cake mix, 1 cup water, ½ cup vegetable oil, 3 large eggs
While the cake is cooling, make the pudding. In a small bowl, whisk together pudding packets and milk until thickened.
6.8 ounces instant vanilla pudding mix, 3 cups milk
Using the end of a wooden spoon, poke holes all over the cake. Spread pudding mixture on top and top, then layer with ⅔ of the banana slices. Cover with plastic wrap and refrigerate at least 30 minutes.
3 bananas
Spread whipped cream on top, then garnish with the remaining bananas and sprinkle all over with vanilla wafers.
16 ounces Cool Whip, 20 vanilla wafer cookies