I live in the USA and want to make Nigella's Birthday Custard Sponge but have no idea what custard powder is or where to find it. Thanks!
Nigella's Birthday Custard Sponge is made with custard powder, which gives it a golden colour and a vanilla flavour. Custard powder was invented by Alfred Bird in the UK in the 1830's. It is added to hot milk to make custard by thickening and flavouring it without using eggs. You can buy custard powder in most UK supermarkets and outside of the UK you may find it in the overseas section of larger supermarkets or from on-line retailers. However, please note that you need the traditional custard powder that is added to milk and not the "instant" custard powder that is made by adding boiling water.
Cornflour (cornstach) is the main ingredient in custard powder, so if you can't find the powder itself we would suggest using 3 tablespoons cornflour plus 2 teaspoons vanilla extract to the cake batter instead of the custard powder. For the buttercream icing use 4 teaspoons of cornflour plus 1 teaspoon vanilla extract instead of the custard powder, adding the vanilla extract after you have added the butter.