Custard:
1. Put custard powder and sugar into a bowl, mix to a smooth paste with of the milk.
2. Put the remaining milk into a saucepan and heat gently until it's almost boiling.
3. Combine the milk and paste mixture and return to heat until it boils and thickens.
4. Allow the custard to cool.
Trifle:
1. Spread lots of your favourite jam on the sponge - as much as you like. Then cut the sponge into fingers and press it into the base of a suitable bowl.
2. Pour the ginger ale over the sponge and add a drop of sherry. Don't drown it, just make sure there is enough to soak the sponge.
3. Pour the cooled custard over the soaking sponge and pop the bowl in the fridge to set.
4. To serve, top with whipped cream, fresh berries or grated chocolate.