Pour brownie mix into a medium-sized bowl and pour in your 12 oz. can of diet coke. Using a hand held mixer or whisk, combine ingredients until you see no more lumps. Pour mixture into a 9 x 9-inch pan coated with cooking spray and bake according to the package directions, adding an additional 5 to 10 minutes to the baking time. Your brownies should bake for about 40 to 45 minutes, or until a toothpick inserted comes out clean. Allow to cool for about 5 minutes and enjoy!