Quite by chance, I just came across the following. I expect you can use
regular Coke if you wanted to:

Diet Coke Brownies

1 box devil's food or chocolate cake mix 10-12 oz. Diet Coke

Preheat oven to 350F. Spray a 9x13 pan with cooking spray. Mix the above
ingredients (must use at least 10 oz. of soda) depending on the
consistency that you want the batter to be. Pour and spread out evenly
into pan and bake about 30 minutes or until you touch it and it comes
back. Let cool completely before you slice it. Makes 24 servings at 2
points each. (weight watchers)

Note: can be made in a 24 mini muffin pan, each muffin is 2
points.