1 cup extra-fine Baker's sugar (regular granulated is fine too)
1/2 cup water
4 egg whites, room temperature
1 teaspoon double vanilla (from Penzey's but I'm sure anything will do)
2 cups butter, cut up into smallish pieces
Combine sugar with water in a sauce pan and bring to a boil. Use a candy thermometer and bring to 235 degrees, which is the soft ball stage.
Using a mixer, whip the egg whites until they get foamy and fluffy. Slowly pour the sugar mixture down the side of the mixer bowl (you don't want to add this directly into the egg foam because it will cook it) while you continue to mix. If you don't have a stand mixer, you'll probably need an extra set of hands during this stage. Add vanilla and check temperature to make sure that it's not too hot. I usually cut up the butter at this point, so that the mixture can have time to cool down a little, otherwise the butter will just melt instead of doing it's thing, and it's sort of horrible if that happens (but not irreversible). Then, while mixing, add the butter one piece at a time to the mix. Don't add another piece until you can't see any evidence of the previous piece. Again, this is where a stand mixer comes in handy because it's usually about five minutes of solid mixing at this point. Then use or refrigerate immediately, because otherwise, I defy you to keep from eating this directly from the bowl with a spoon.I almost never make cakes from scratch, because box mixes are usually better than homemade (sorry Martha, but they are) however, homemade frosting is de rigeur and entirely worth the effort. Butter cream is the version that all other frostings bow before.