Preheat oven to 350 degrees.
Grab mixing bowl. Add dry ingredients. Mix.
Slowly add Diet Coke. Mix until smooth.
Grab an 7x7 or 8x8 baking dish. Spray with cooking spray generously or add parchment paper. Add batter and spread evenly.
Bake for 14-18 minutes at 350 degrees. Take out once the center is just set so it stays soft and fluffy. Make sure to see if toothpick comes out mostly clean.
Let brownie cake cool completely.
Grab a small bowl. Add frosting ingredients. Mix until you get a thick frosting consistency.
Spread frosting evenly over brownie cake.
Add optional Cosmic Brownie candy chips or rainbow nonpareils on top.
Cut into 9 brownies and enjoy!
