Butter cream is a wonderful goo full of fats and sugars used as frosting, icing and garnish. Many different goos are called butter cream : There are (at least) two kinds of American butter cream (both of which are simple, cheap and not so yummy) and three types of French butter cream (all of which are a lot of work and really yummy). All butter creams may be flavored with, vanilla, citrus zest, liqueur, mocha, praline, chocolate or pretty much anything else that doesn't contain much water. I have no direct experience with the two types of French butter cream with custard, but they sound good and I'll paraphrase from Larousse Gastronomique.
The industrial American butter cream recipe combines something like two parts powdered sugar to one part Crisco. It lasts forever, keeps its shape well, and pleases most palettes under age six. It's what you'll find on a cake you get real cheap at a drug store.
The home American butter cream recipe combines powdered sugar with butter and cream, and is pretty much as described in butter cream icing.
French butter cream with syrup, is just about the best thing you can put in your mouth that isn't part of someone else. It is made thus :
  1. Boil two cups of sugar in one and a half cups of water, until all the water is gone, or about 220o F or 110o C.
  2. Beat some egg yolks (two to twelve, depending on how rich your taste) in a mixer, at a high speed.
  3. While the mixer continues to mix, slowly add the hot sugar syrup.
  4. Turn the mixer speed down a bit, and continue mixing until it has cooled. You can accelerate the process by slipping an ice bath under the mixing bowl.
  5. When you added the syrup to the yolks, you beat quickly and added slowly to prevent any lumps of cooked yolk from forming. If you have lumps, you might want to insert a straining step here.
  6. Slowly add a pound of room temperature butter to the cooled mixture.
The only trick here is that sometimes the butter cream "breaks", meaning instead of being smooth and firm like butter it is lumpy and runny like cream of wheat. Breaking can be caused by excess water or the wrong phase of the moon. Sometimes adding a little flour and beating some more will un-break it, sometimes it won't. Broken butter cream is still quite yummy, just not as attractive or spreadable.
French butter cream with custard version I
  1. Make custard cream a.k.a. pastry cream as for filling eclairs.
  2. Beat in a pound of butter for every three cups of custard.

French butter cream with custard version II
  1. Combine 12 egg yolks, 1 pound fine sugar, one pint cream and flavoring.
  2. Mix until smooth.
  3. Whip over gentle heat until it becomes frothy, light and whitish.
  4. Remove from heat and continue to whip until cool.
  5. Cream a pound of butter separately, and mix well into the other mixture.

sources : personal experience my ex-wife Larousse Gastronomique