Preheat the oven to 350°F and lightly grease a 9×13-inch baking pan with nonstick spray.
Prepare the yellow cake mix according to package instructions and pour into the prepared pan.
Bake for 30 minutes or until a toothpick inserted in the center comes out clean.
Use the handle of a wooden spoon to poke holes all over the warm cake.
In a large bowl, whisk together the banana pudding mix and milk for 2 minutes until smooth but still pourable.
Pour half of the pudding mixture over the warm cake, pressing it gently into the holes.
Spread the remaining pudding evenly over the top of the cake.
Layer thinly sliced bananas evenly across the pudding layer.
Spread the whipped topping evenly across the top and garnish with vanilla wafers.
Refrigerate the cake for at least 12 hours or overnight before serving.
