Preheat your oven to 350℉ (175℃) and grease a 9x13-inch baking dish with nonstick cooking spray.

In a large mixing bowl, combine the instant banana pudding mix and milk. Whisk vigorously for about 2 minutes until smooth and slightly thickened.

Add the yellow cake mix to the pudding mixture and mix until well combined and smooth, being careful not to overmix.

Gently fold in the thinly sliced bananas, distributing them evenly throughout the batter.

Pour the brownie mixture into the prepared baking dish and sprinkle chopped vanilla wafer cookies on top.

Bake for 30-35 minutes until the brownies rise slightly and a toothpick inserted comes out clean or with a few moist crumbs.

Remove from the oven and cool in the pan for at least 15-20 minutes before slicing.