Coffee and Walnut Cake Frosting

Our answer

Nigella's Coffee And Walnut Cake (from Kitchen and on the Nigella website) has a coffee-flavoured buttercream that is a simple buttercream, made from butter, icing sugar (confectioners'/powdered sugar) and coffee powder. In the UK icing sugar is very finely ground and should give a smooth buttercream. We have heard suggestions that if maize starch is used as an anti-caking agent in the icing sugar then this can give a slightly gritty buttercream, though we have not experiened this ourselves.

Unfortunately if you live in some parts of Europe the powdered sugar may not be quite as finely ground as the UK's icing sugar and can be a litte more grainy. In the US confectioner's/powdered sugar does have 3 grades. Usually in the supermarket it is the very fine "10x" but if you bought the sugar from a specialty baking store then you may have inadvertently bought "XXXX" or even "XXX" sugar which are coarser. The grade should be marked on the packaging. The lumps in the sugar are usually caused by moisture rather than an indication of the fineness of the sugar. Rub some sugar between your fingers to see if it feels smooth or slightly gritty. If it is gritty and you have a food processor then blitz the sugar for a couple of minutes before adding the remaining ingredients. This should help to grind the sugar a little more finely, as well as removing any lumps.