Hi there. I'm making a double layer cake for my daughter's birthday. I'm going to use the Buttermilk Birthday Cake recipe as I know it will work for me (and hold up to fondant icing), but I was wondering if I could make one of the layers chocolate as my daughter really wants chocolate cake. How best would I go about this? Substitute some flour for cocoa? Add melted chocolate? Any advice would be helpful!
It would be possible to substitute some flour for cocoa powder in a cake such as Nigella's Buttermilk Birthday Cake (from Domestic Goddess) but cocoa has slightly different properties to flour and you may need to add some extra leavening. We suspect you could replace 50g(1/3 cup) of the flour with 50g (1/2 cup) unsweetened cocoa powder would need to increase the baking powder from 1/2 teaspoon to 1 teaspoon and the bicarbonate of soda from 1/4 teaspoon to 1/2 teaspoon. However we have not tested this and so cannot guarantee results.
Instead we would suggest trying one of Nigella's other chocolate cake recipes. The Old Fashioned Chocolate Cake from Feast has a fairly similar recipe to the buttermiilk birthday cake and you should be able to use buttermilk instead of sour cream in the cake batter. The Devil's Food Cake from Kitchen has a very good chocolate flavour and is also a cake that cuts well and would be able to take the weight of a fondant icing.