Basic, Swiss buttercream—fast and simpler. It's from my daughter, Andrea. However, the one on the right is the one I use more often. It's more or less a half recipe and just the right amount for a store-bought cake mix even when done in three cake pans (instead of the usual 2).
1. Heat over bain-marie until 85° Celsius (185° Fahrenheit). I find this too hot and it leads to cooking the egg whites. Instead, I stop anywhere over 140° Fahrenheit. Really, I just poke it with my finger and if it's pretty hot, I stop.
2. Whip until room temperature. I rub the sides of my KitchenAid® bowl with handfuls of ice cubes or plunge the bowl (carefully) into a sink with enough cold water to go up the side (or I do a little of both).
3. While still whipping, slowly add the butter until smooth. I like my butter colder to compensate for the heated egg white, but also because I often make chocolate buttercream. If too cold, the buttercream will be grainy. No matter, just beat longer or let it warm up a bit and beat again. Buttercream can be very forgiving.
4. Add vanilla to taste; can add slightly melted chocolate. (I melt mine in a bain marie with a couple of tablespoons of butter, then set it aside to cool while I'm making the rest of this recipe.) I compensate for the temperature of the chocolate by leaving some of the butter at refrigerator temperature, but this sometimes makes me beat the frosting longer as lumps of butter persist.
Store in refrigerator tight against the depredations of oxygen. It will last even longer in freezer, up to a couple of months.
This complicates the recipe. If you put the cream cheese in with the butter, you'll surely get a grainy mess. The secret is to incorporate cream cheese after the buttercream is made and only once the cream cheese has sat on the counter (outside the refrigerator) until it reaches the same temperature as the buttercream.
My daugher recommends:
A seasoned chef would know much more. However, here are some experiences I have lived through and what I did to save the buttercream.
Never expect egg white to beat into foam if there is the least little bit of fat present.
Egg white is about 90% water plus 10% egg protein. The reason you can make foam for meringue or other purposes is because the proteins, really amino acids, will link up, stick together and tend to form at the microlevel on the outside around trapping air and water. You can add more acid, the form of cream of tartar, to egg whites to enhance this natural reaction.
When you contaminate the emulsion with a fat, such as a bit of egg yolk, or trace amounts of residual fat left over from something else you've used the bowl for (why you should never make meringue in a bowl that's plastic or another, porous material like wood), the fat defeats the structural mechanics of protein encapsulating air and water in the foam.
Do not try to use the chocolate in the buttercream. If you add butter and cream to it, you may succeed in reliquifying it to make a chocolate sauce.
Chocolate appears greasy to us, but at the micro level, it's just a powder, like flour. Just as letting a few drops of water into flour will ruin it by turning it into mud—it must be something you really want to do on the way to where you're going with the flour, chocolate cannot endure this treatment and making a dough is absolutely not something you want to do on the way to buttercream.
The real solution is to beat more, but the temperature of everything (the butter, the egg whites, the bowl, the air around you) may not cooperate or is too hard (too late) to control.
Another solution is to put the bowl into the refrigerator for half an hour and beat again. It especially happens when the egg whites didn't beat into foam well.
Often, if you haven't added chocolate or cream cheese (that has warmed to room temperature and been prebeaten) yet, do that and it will probably serve to melt/soften the butter pieces, then beat the whole as fast and hard as you can. Try refrigerating it and beating again.