1. Heat oven to 350°F. Spray a 13 x 9 inch pan with cooking spray.
2. In a large bowl, beat cake mix, water, oil, eggs and mashed bananas with electric mixer on medium speed 2 minutes, scraping bowl as needed.
3. Bake 32 to 35 minutes or until toothpick inserted in center comes out clean. My oven cooks quick so normally 32 minutes is perfect. Cool in pan for 5 minutes.
4. While cake is cooling, whisk cold milk and pudding together in a medium bowl until pudding is soft and thickened, about 2 minutes.
5. Using the handle of a wooden spoon, poke holes in cake, almost to bottom of pan, about every 1/2 inch.
6.Pour pudding over cake ; spread evenly, working back and forth to fill holes as much as possible. Some filling will remain on top of cake. Refrigerate at least 4 hours if you can. I like to make my cake the night before or the morning of when I need it.
7. When ready to serve top with sliced bananas, cool whip and Nilla wafers.