To make the lemon custard, combine custard powder, sugar, rind and enough milk to make a smooth paste in a saucepan.
Stir in remaining milk over heat until mixture boils and thickens. Allow to cool.
Make cake according to instructions on packet using eggs, water and butter.
Line 2 x 12-hole patty pans with the paper patty cases.
Spoon half the mixture into patty cases. Top each with 2 level tsp of lemon custard.
Spoon over remaining cake mixture to cover custard.
Cook at 180C for about 20 minutes, or until cooked through. Cool on a wire rack.
Dust cakes with sifted icing sugar to decorate.
