When I make the icing for the Old Fashioned Chocolate Cake it always seems to be a little runny. I have to put the icing in the fridge now and then when icing the cake. And it slipped down the sides of the cake as I iced it. Is there any possible way I could fix this? Thanks!
Nigella's Old Fashioned Chocolate Cake (from Feast and on the Nigella website) is a very popular cake and the icing should be soft enough to spread, but not runny. It is possible that the melted chocolate and butter mixture has not cooled enough before you add the other ingredients which will make the overall mixture a little runny, so let the melted chocolate and butter cool sufficiently, to cool room temperature. Also use regular/full fat sour cream, not a low-fat version or a thinner pouring cream.
Also make sure that the cake is cold before icing it. If the cake is still warm then the icing will warm up a little as you apply it and could run off the cake.
You can add some extra icing sugar (confectioner's sugar) to the icing to thicken it. Otherwise refrigerate the icing for up to 10 minutes, stirring it every 2 minutes, until it has reached a thick, but still spreadable, consistency. You do need to be careful doing this as the icing can become firm quite quickly once it has cooled down.