Is it possible to make Nigella's Birthday Custard Sponge a day in advance of serving and if so does it need to go in the fridge? I am a little worried in particular about the icing. Also, when the recipe says 'eggs', what size? Thanks.
It is possible to make most of the cake a day in advance, though we would suggest for the best results that the chocolate icing on top is added 4 to 6 hours before the cake is served. Both icings are able to stand in a cool place and don't have to be kept in the fridge and leftovers will keep for 2 to 3 days in an airtight container.
You can make the sponge part of the cake and the buttercream filling the day before. Sandwich the cake with the filling and store in an airtight container overnight then cover with the chocolate icing the following morning. You an also make the sponge and buttercream up to 3 months in advance and freeze them. Wrap each sponge layer in a double layer of clingfilm (plastic wrap) and then a layer of foil and freeze. Make the buttercream, transfer it to an airtight container and freeze. The sponge layers will take a couple hours to thaw at room temperature. The buttercream is best thawed overnight in the fridge then let it stand for a couple of hours at room temperature, until it is soft enough to spread (you can beat it briefly again before using it).
Nigella's recipes usually use UK (and European) large eggs, For the US, Canada and Australia this is equivalent to extra large size.