For the icing/frosting on Nigella's Old Fashioned Chocolate Cake can I use mascarpone instead of sour cream?
The chocolate frosting on Nigella's Old fashioned Chocolate Cake (from Feast) is a type of ganache, made with melted butter, sour cream and chocoate. Whilst theoretically mascarpone could be used in a ganache, we would prefer not to use mascarpone in this icing as the fat content is very high, at around 40%, and this will make the frosting very rich. Also mascarpone is very thick so may not incorporate into the melted butter and chocolate as easily as sour cream, you may end up with small lumps of mascarpone.
The icing also contains a lot of icing sugar (confectioner's sugar) and the slightly acidic tang of sour cream helps to cut the sweetness and richness of the icing. The best alternative for the sour cream would be low fat or reduced fat cerme frache (which usually has a fat content of around 14%, very similar to sour cream) or regular creme fraiche (which has a fat content of around 25%). Yogurt is not a good substitute in this recipe as the fat content is too low and it can be a little unstable in this type of recipe.