I have noticed some recipes use self-raising flour, could I use self-raising in Nigella's recipe for Chocolate Guinness Cake instead of plain?
There is quite a difference between self-raising flour and plain (all-purpose) flour and we would not recommed using self-raising as an alternative to plain flour in Nigella's Chocolate Guinness Cake (from Feast and on the Nigella website) or in any other recipes. One of the problems is that self-raising flour contains raising agents (and in the US salt is also added) in fixed quantities and many recipes which use plain flour have raising agents added in quantities that are unique for that paricular cake.
In the Chocolate Guinness cake bicarbonate of soda (baking soda) is used as the raising agent as it reacts well with the acidity of the Guinness to give cake a good rise and particular texture. If you use self-raising flour and bicarbonate of soda then the cake will rise up too much and may spill out of the pan, it will then sink dramatically as it cools. If you omit the bicarbonate of soda than it may not rise enough and be too dense in texture.