Preheat oven to 350 degrees F. Liberally grease a 9x13" rectangular baking pan with cooking spray. Prepare the cake mix according to the package directions and pour the batter into the prepared pan. Bake for approx. 20-25 minutes or until a toothpick inserted near the center comes out clean or with moist, not wet, crumbs. Cool for about 30 minutes.
Poke holes all over the cake with the end of a wooden spoon or using a skewer or fork. Meanwhile, in a large bowl, combine the pudding mixes and cold milk until combined. Pour the banana pudding mixture evenly over the holes in the cake. Refrigerate for at least 1 hour.
Place the sliced banana coins evenly over the top of the pudding layer on the cake. To make the whipped cream, in the bowl of a stand mixer, beat the whipping cream and confectioners' sugar on HIGH speed for about 5-7 minutes or until stiff peaks form.
Spread the whipped cream evenly over the top of the cake and sprinkle with the crushed Nilla Wafers. Garnish with additional bananas just before serving. Store covered in the fridge, up to 2 days.