Heat the oven to 350°F and grease a 9 x 13-inch baking dish. Prepare the yellow cake mix according to the package directions, usually mixing in 3 large eggs, 1 cup water, and 1/2 cup vegetable oil.

Spread the batter evenly in the prepared baking dish. Bake for 28–33 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool for 5–10 minutes. While it is still warm, poke holes all over the cake with the handle of a wooden spoon or a large straw.

Whisk the two boxes of instant banana pudding mix with 4 cups of cold milk for 1–2 minutes, until thickened. Pour the pudding over the warm cake right away, spreading it across the surface so it flows into the holes.

Chill the cake for at least 1 hour so the pudding can set and the cake can cool completely. Spread the thawed whipped topping evenly over the chilled cake.

Slice the 3 medium bananas and arrange them over the whipped topping. Finish with about 20 crushed or broken vanilla wafer cookies just before serving.