I made Nigella's Old Fashioned Chocolate Cake but the icing turned out to be grainy. I added a little water but nothing happened. I even mixed it properly but its grainy texture remained
Nigella's Old Fashioned Chocolate Cake (from Feast) is iced with a frosting made from melted butter and chocolate, icing (confectioner's) sugar, golden syrup and sour cream. There are two ingredients in this list that can turn grainy if over heated, one if the chocolate and the other is the sour cream.
It is most likely that the chocolte turned grainy, or "siezed" as it was being melted. One of the reasons could be that the chocolate became over heated. The bowl of chocolate and butter should be suspened over a saucepan of hot water, and the base of the bowl should not touch the water. Also the water only needs to be hot, not boiling or simmering too briskly. Stir the mixture regularly as the chocolate melts, and take the bowl off the saucerpan if it looks as if bits of the chococlate are turning grainy. You should also be careful that no drops of water get into the bowl as the water can cause melted chocolate to sieze.
Make sure also that the chocolate mixture has cooled a bit and that the sour cream is not completely fridge cold when you mix the two together. If the chocolate mixture was very hot then the sour cream could also separate and turn grainy. Adding a very cold cream to melted chocolate can also sometimes upset the balance and also cause the chocolate to sieze and turn grainy.