Q&A for How to Make Buttercream Filling

There's American-style buttercream, which is a mixture of butter, powdered sugar, and milk; Swiss meringue buttercream, which is a mixture of egg whites, sugar, and butter; Italian meringue buttercream, which is a mixture of sugar syrup (sugar dissolved in water), egg whites, and softened butter; and French buttercream, which is a mixture of egg yolks, sugar, and softened butter.

You can, but when it thaws the buttercream filling may separate. If it does, it may become watery and this will make the cake soggy. You can freeze buttercream icing by itself for up to three months in an airtight container. Just give it a stir after it thaws completely before using it.

You could use confectioner's sugar. This actually has less sugar in it, but it does have other additives that might affect the texture of your buttercream filling.

It will be fresh for 2-3 days, after that it will start to harden up.

You could use extra-fine sugar, but the filling would be really grainy.

No. The artificial flavor used in flavoring margarine would become very pronounced, and margarine will contribute to a softer texture bordering on melting off your cake. Butter-flavored shortening would contribute to better texture, but the taste will still be compromised. You might consider an icing instead if you are trying to avoid the use of butter.