When I make chocolate icing it often is tasty, but isn’t as smooth and glossy as I’d like. Well, I think that I’ve found the cause, as well as the solution, in a 1919 magazine article.
Why Chocolate Icing Loses Its Gloss
If a chocolate icing is beaten too much before spreading, the gloss will be lost. It should be spread while it is yet a little “runny,” so that it flows of itself to a great extent over the surface of the cake. Sometimes if a knife-blade, dipped into hot water is used to smooth the icing, it will restore the gloss.
American Cookery (December, 1919)