Dear Nigella Team, I recently made Nigella's coffee cake but I was out of instant of espresso so I used Camp coffee essence and it curdled. Is this normal? The cake batter curdled and so did the icing. Thanks, Jack
We are not sure if you are referring to the Victoria Sponge coffee variation from Domestic Goddess (p14) or the Coffee and Walnut Layer Cake from Kitchen (p275) but in either case we doubt that the coffee essence was the cause of the curdling in the cake batter. Before strong coffee powder was available cooks used to use coffee essence to flavour cakes so its use in cake making is well-established. If you are making cakes via the "all in one" method then the coffee should just be added along with the other ingredients. If you are making the cake by a more traditional creaming method then the batter usually splits when the eggs are being beaten in so add a little of the flour with each addition of egg to help keep the mixture together and add the coffee essence after all of the the flour has been mixed in.
Simple butter and icing (confectioner's) sugar buttercreams can very occasionally split when liqiids are added, especially if the liquid being added is cold. Make sure that the coffe essence is at room temperature and if you add any milk to soften the icing then make sure it is at room temperature too. You can often rescue a split buttercream by beating in some extra icing sugar. Icing sugar usually contains a little cornfour and this should help to stabilize the buttercream.