As others have said, this is not frosting, but a separate deliciousness in and of itself. Added on top of the brownies after baking, or swirled into the batter before baking (unlike mixing it in entirely like BtES said) would add a nice contrast and texture.
Frosting, however, is just sugar, fat, and flavoring. It is just fine on many foods, but for brownies it is overkill.
I have a recipe for peppermint patty brownies that would be ruined by frosting.
I have a recipe for black forest brownies that are ok, but lacking without the frosting.
Oh, and lets not forget the caramel brownies, where again frosting would be ruinous!