Can self-raising flour be used instead of plain flour in Nigella's Old Fashioned Chocolate Cake recipe? Also does it have to be 'unsalted' butter for this recipe?
Whilst it is quite easy to substitute self-raising flour with plain flour in a recipe - by using plain (all-purpose) flour plus 2 teaspoons baking powder per 150g (1 cup) flour - it is much more difficult to switch plain with self-raising flour.
The reason for this is that usually if plain flour has been used, the raising agents in the recipe are in a different proportion to to those in self-raising flour. Nigella's Old Fashioned Chocolate Cake (from Feast) uses a combination of baking powder and bicarbonate of soda (baking soda) which cannot be easily adjusted for self-raising flour so we would not recommend this switch.
Most bakers prefer to use unsalted butter in recipes as it gives better control over the salt content of the cake. However would be possible to used salted butter in the cake batter if you prefer. But we would not recommend using salted butter in the icing as it will be much more noticeable and could be a little too salty.