Preheat your oven to 350°F. Lightly coat a 9x13-inch baking pan with nonstick spray and set aside.

In a large bowl, combine the yellow cake mix with the water, vegetable oil, and eggs as directed on the box. Whisk by hand for about 1 minute until smooth and no lumps remain.

Pour the cake batter into the prepared pan and bake for 30 minutes, or until a toothpick inserted in the center comes out clean.

While the cake is still warm, use the handle of a wooden spoon to poke holes all over the cake. Let it cool slightly on the counter.

In another large bowl, whisk the instant banana pudding mix with the cold milk for about 2 minutes until combined. Do not let it set fully; it should be pourable.

Pour half of the pudding mixture over the warm cake, making sure it fills the holes. Gently press with the back of a spoon if needed.

Pour the remaining pudding over the cake and spread it into an even layer.

Arrange the sliced bananas in a single layer over the pudding.

Spread the thawed whipped topping evenly over the bananas.

Place the vanilla wafer cookies on top, one per serving area. Refrigerate the cake for at least 12 hours or overnight before slicing and serving.