I would like to try using just instant custard in between the sponge layers instead of buttercream for the Birthday Custard Sponge. Is this possible or would the sponge become soggy?
Nigella's Birthday Custard Sponge (from FEAST) is a victoria sponge cake that is flavoured with custard powder. The custard powder also gives the sponge a golden yellow colour. The buttercream filling is also flavoured with custard powder.
We would not particularly recommend using custard (creme anglaise) on its own as a filling for the cake. If you used a ready made or instant custard (this is the type that comes in a sachet and is mixed with boiling water) then it would need to be a particularly thick version, otherwise the custard will squidge out of the sides of the cake as you sandwich the layers together and also as you cut the cake. The custard would need to be well chilled and only used just before the cake is served. Leftovers would need to be refrigerated and eaten quickly, as the custard will turn the bottom of the cake soggy as it stands. If you can't find custard powder then substitute 3 tablespoons of cornflour (cornstarch) plus 2 teaspoons of vanilla extract in the sponge batter and 4 teaspoons of cornflour plus 1 teaspoon of vanilla extract in the buttercream filling.