Preheat oven to 180C (350F).
Combine flour and 1/4 cup of the caster sugar in a bowl
Add egg, butter, milk and lemon rind and beat to combine.
Pour into a 6-cup capacity ovenproof dish.
Combine cornflour, custard powder and the remaining 1/2 cup of caster sugar and sprinkle over top of pudding.
Combine lemon juice with 11/2 cups boiling water and pour over pudding.
Bake pudding for 30-40 minutes or until puffed and golden.
Dust with sifted icing sugar and serve with ice-cream or yoghurt, if desired.