Most of the time I make my buttercream with a Swiss meringue (sugar and egg white whisked together over boiling water untille sugar is dissolved). I never have proplems with this icing but it takes a long time to make. I made Nigella's Malteser Cake (Feast, p283). Here the buttercream is made with butter and icing sugar (powdered or confectioners' sugar). I've tried buttercream with icing sugar already a lot of times but everytime I have the same problem - and again with the Malteser Cake. The recipe states that you should mix until smooth. But how long is this - I leave it for quite a while in a stand mixer and still everytime the buttercream has kind of a crunch. Like the powder sugar has crystallized, Have I beaten it for too long? Is it supposed to have this granulated texture? The taste is great but the texture is quite strange.
A simple buttercream made with just icing sugar and butter should not have a grainy texture. It is difficult to say exactly what is causing this texture without seeing the exact method but our guess is that the icing sugar had some lumps in it and these have not fully dissolved as stand mixers can have large mixing paddles, giving the slightly grainy texture. It could also be that the texture is actually not sugar but comes from lots of small air bubbles which may have formed if the buttercream is beaten for a long time.
Nigella likes to make her buttercream in a food processor as the blade of the processor breaks up any lumps in the sugar, ensuring a smooth buttercream. If you are making the buttercream in a stand mixer (or with a hand-held mixer or by hand) then sift the icing sugar after measuring it out, to remove lumps, and beat the sugar into the butter in 3 or 4 additions rather than all at once. You do not need to beat buttercream for a long time, only until the butter and sugar have combined, which should only take a couple of minutes. Add any flavourings or colours at the end and again only beat until they are just combined.