For the sponge:
100 ml flour
150 g sugar
100 g butter
3 eggs
2 tablespoons grapefruit juice
Half grapefruit grated rind
1/2 tablespoon baking powder
For the syrup:
5 tablespoons water
100 g sugar
1 glass orange liqueur
1/2 kg custard

Put the butter with the syrup in a bowl and beat vigourously. Add the eggs, do not stop beating. Add the grated rind and the grapefruit juice; pour the mixture in a greased mould with butter, until covering half of the mould. Bake for 45 minutes. Remove from its mould and allow to cool on a grill. Prepare the syrup with the water and the sugar, flavour with the orange liqueur. Put the sponge in a dish, soak with the syrup and add the custard before being served.