This preparation occupies a sort of middle ground between a coating and a frosting. Whatever you call it, it’s good. Begin by putting your chocolate in a microwave-safe bowl. This is mostly milk chocolate chips, though since I had a handful (about an ounce) of bittersweet chips in a mostly-empty bag, I threw those in too.
Apply ten seconds of full power, then stir. Apply another ten seconds. Stir. Continue like this until your chocolate looks about like so, then stop. Stir it the rest of the way until it’s completely smooth. It’ll take a few minutes.
When the chocolate is completely melted, let it cool until it’s barely warm to the touch. Which is to say, about as cool as you can get it while still having it flow. Put it in a bowl with the soft butter…
…and beat about a minute until smooth, scraping once or twice. Now here I should mention that since I had no heat in my house this weekend, my implements and bowl were very cold. The upshot was that some of the chocolate solidified the instant it hit the metal, creating chunks of solid chocolate in the frosting. What did I do? I soaked a small kitchen towel in hot tap water and applied it to the sides of the bowl as the machine ran. The small amount of heat warmed the mixture enough that the lumps melted out. Since that action also warmed (and thinned) the frosting, I beat it an extra minute to cool it down.
Then I promptly applied to the cake. It firmed immediately in my 50-degree house.
Normally I like to work with frozen cake layers. However for reasons that should be fairly obvious right now, you can’t do that with this kind of frosting. Have the layers at or close to room temperature as you build the cake, and apply the frosting quickly since it will set up fast…especially in a house with a broken furnace.