They share a few of the same ingredients: butter, cocoa, and powdered sugar. Heavy cream (or milk) is added to buttercream and a lot more powdered sugar to make sweet frosting. Chocolate butter is not used as frosting and wouldn't hold its structure the same way that buttercream does because it has less powdered sugar. Since buttercream is a frosting, it would be probably too sweet to spread on your breakfast toast but chocolate butter is just right!