For chocolatizing a cake, there's ganache, there's the custard-like whipped chocolate frosting, and there is this. They are so good I have stopped experimenting with anything else.
This tastes like the boardwalk fudge of my youth, rich and dark with a sneaky salty bite that makes you need more. It pairs beautifully with elegant white vanilla cake, as well as any lighter chocolate cake, such as devil's food (it's overkill on anything that's similarly dark-chocolatey).
This, and pretty much any butter cream-style frosting, can be made weeks ahead of time and kept in the fridge or freezer. The worst that can happen is that it will be hard to work with when cold (solved by time, or, if desperate, microwave) or it may dry out a tad (solved by a sprinkle of milk or water).
This recipe makes enough to fill and frost a three-layer cake. That's perhaps 8 cups. Half it if you have more reasonable frosting needs.
Melt the chocolate, by short bursts in the microwave if you're brave; by slow stirring in a saucepan over the lowest possible heat, if you're me. NOTE: Take care that all dishes and utensils are bone-dry. A drop of water will cause the chocolate to seize (go irreversibly grainy and terrible), and then you're fucked. Don't be paranoid but it's good to stay nervous.
Set chocolate aside to cool a bit while you sift the sugar, which takes forever.
Cream together butter, vanilla, and salt. Mix in about half of the sugar.
Alternately add some of the chocolate, then some of the remaining sugar, until both are in there. The mixture may look dry or chunky - that's fine.
Add the milk slowly, and you may not need quite all of it. Alternately, you may need a little more than what's called for. Stop when the frosting is a spreadable consistency. It will firm up a little as it cools.
Add the coffee element, if using. See note below about relative mocha-ness. Add extra salt, coffee, and/or vanilla, by very small amounts, until it's amazing.
Sift the sugar or you'll regret it.
Like everything else, this recipe reacts to the relative humidity of your kitchen. I don't think I have ever once used the exact amounts of sugar and milk I measured out ahead of time. This frosting is very forgiving and mess-with-able. If you get to the end and find the consistency all wrong, add more wet or dry ingredients as needed, an it will turn out fine as long as you taste it at the end and make sure the salt and other flavors are still strong enough. (The chocolate-ness will take care of itself.)
You can take this to a mocha level, if you like. If your coffee flavoring measures more than a couple teaspoons, you might want to reduce the amount of milk by an equivalent amount, or just plan on adding more sugar - it's flexible. Espresso powder, being dry, eliminates this moisture difference, and gives you more freedom to play. (Instant coffee will work, in a pinch.)
If a mocha flavor's not what you're after, I do still suggest using a small amount of coffee flavoring. It's not detectable in the final product (I have tested this on coffee-hating friends), but just serves to deepen the chocolate flavor and send it in a fudgey direction somehow.
I have not experimented, but I feel like this would be killer if you used orange or mint extract in place of the coffee. Still needs the vanilla though.