(Frozen) Grandma's Buttermilk Brownie w/ Chocolate Buttermilk Frosting

This brownie is more like a moist chocolate cake. Buttermilk gives it a unique and wonderful flavor. Grandma made these all throughout Whitney's childhood. She was an amazing midwest small town home cook and baker.
 
Ingredients: grade AA butter, buttermilk,  cocoa, organic eggs, sugar, canola oil, powdered sugar, flour, baking soda, vanilla, salt