The crème au beurre, aka buttercream, is one of the basic preparations in pastry: it is a light and smooth mixture of fat and sugar and it can be used as filling and decor as well.
Buttercreams are popular for many American- and English-style cakes (as filling, decor and levelling layer between the actual cake and the icing), for cupcake frosting, but also for some French classics like the Moka cake, Religieuses or the Christmas Yule Logs (bûches de Noël).
There are several preparation methods for buttercream, but they all have one step in common: creaming the (soft) butter to incorporate air and make the final cream lighter. Then pasteurized eggs (i.e. “cooked” with a sugar syrup) are usually added, either whole, or just yolks (for richness) or whites (for lightness).
The main different methods are:
- Simple American buttercream: made by creaming the butter and powdered sugar until light and fluffy. Pasteurized eggs and flavours can be added optionally. It is very easy to make and for this reason it is widely used, but it has a strong taste which might or might not be appropriate for your recipe.
- French buttercream: egg yolks are whipped then pasteurized with a hot sugar syrup (aka “pâte à bombe“); only when the mixture has cooled down the soft butter is added and creamed. Probably the most difficult method to master, but the one with the best flavour and smoothest texture. An Italian meringue can be added to the buttercream for additional lightness.
- Italian buttercream: in this case, only egg whites are used to make an Italian meringue; when the mixture has cooled down, soft butter is added and creamed. The meringue adds lightness to the cream
- Swiss buttercream: it is similar to the Italian buttercream, but the Italian meringue is replaced with a Swiss meringue; this means that egg whites are first heated and lightly whipped with sugar on a bain-marie, then whipped in a mixer until they cool down; finally, butter is incorporated and creamed to make a light and glossy buttercream.
- English buttercream: the butter is added to a mixture which is similar to a custard sauce (Crème anglaise) with egg yolks, sugar and milk)
- Genoise-style buttercream: it is similar to the French buttercream, but whole eggs are used instead of just yolks.
Of course there is also a way to make buttercream with raw eggs (i.e. not pasteurized), but due to health and safety concerns, it is rare to find it in commercial bakeries and is probably better to avoid it anyway!