Preheat the oven to 350 degrees F (175 degrees C).
Lightly grease a 9x13 inch casserole dish.
Place the yellow cake mix in a large mixing bowl.
Add the eggs, water, and vegetable oil to the bowl.
Beat the mixture until the cake batter is smooth.
Pour the batter into the prepared casserole dish and spread it evenly from edge to edge.
Place the casserole dish in the preheated oven.
Bake for 28 to 32 minutes, or until the center is set and a toothpick inserted in the middle comes out clean.
Remove the casserole dish from the oven and place it on a cooling rack.
Cool the cake for 20 minutes. This prevents the pudding from melting or becoming runny.
Poke holes across the entire cake using the handle of a wooden spoon. Space the holes about one inch apart and push deep into the cake without tearing the bottom.
In a large mixing bowl, combine the instant banana cream pudding mix and cold milk.
Whisk the mixture for 2 minutes until the pudding begins to thicken.
Pour the pudding evenly over the cake, spreading it gently to fill the holes. Use a spatula to guide the pudding into each opening.
Cover the casserole dish and refrigerate for at least 2 hours. This allows the pudding to set and soak into the cake.
After 2 hours, remove the cake from the refrigerator.
Arrange the banana slices evenly over the pudding layer.
Spread the whipped topping evenly over the bananas. Work gently so the bananas stay in place.
Sprinkle the crushed vanilla wafer cookies over the top.
Cover the casserole dish and refrigerate for 1 additional hour.
Finally, remove the cake from the refrigerator and cut into 12 portions.
Serve chilled, and enjoy the delicious banana cream pie poke cake.