Preheat oven to the temperature specified on your cake mix.
Make and cook cake as directed on your cake mix box for a 13x9-inch pan.
Remove cake from the oven and let it cool in the pan for at least 5 minutes
With handle of a wooden spoon poke evenly spaced holes almost to bottom of cake, about 1/2 to 1 inch apart.
If you wipe the wooden spoon handle in between it will help stop it sticking and pulling out lumps of cake.
In a bowl mix together the milk and banana pudding as directed on the pudding packet.
When the cake is fully cooled pour mixed pudding over the cake and spread evenly over surface to get it into the holes and fully cover the surface of the cake, then refrigerate for 1 hour, this will allow the pudding to set enough to easily add the whipped topping.
Spread whipped cream or whipped topping evenly over the pudding layer on the cake.
Just before you serve it add crumbled Nilla wafers and then sliced bananas over the top, the bananas will turn black if you cut them too far ahead of time.
The instant pudding is very simple to make and will start to set right away. It will set up better and make it easier to apply the whipped cream or whipped topping if after you've added it onto the cake, you then chill it in the fridge for about an hour. After you've added the whipped topping you can keep the cake covered in the fridge until you are ready to serve it.
Once you're ready to serve you can add the crushed wafers and the sliced bananas, if you put the bananas on it too far ahead of time they will eventually start to turn brown.