Preheat oven to 350 degrees. Spray a 9×13-inch baking dish with cooking spray; set aside. In a large bowl, combine the dry cake mix and the Diet Coke. Whisk together until just combined. Do not over mix. Bake for 25-28 minutes or until a toothpick inserted in the center comes out clean. Cool completely.For the frosting: Whisk together the dry pudding mix, milk and vanilla until combined. Fold in half of the Cool Whip and then fold in the remaining half of the Cool Whip. Spread over cooled cake. Garnish with chopped Oreos, if desired. Cut into 15 squares and serve. Keep leftover cake stored in the refrigerator.
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