1. Mix the sugar, flour and salt in the top of a double boiler. Add the milk gradually, stirring. Cook, stirring, over boiling water until thickened.
2. Beat the egg yolks until blended. Gradually add part of the milk mixture to the yolks, while stirring. Return to the hot mixture and cook over simmering water, stirring constantly, until thickened. Cool and chill.
3. Divide the mixture into three parts. Add one tablespoon rum to one part, the vanilla to the second and the creme de cacao to the third.
4. Split the sponge cake layers to make four layers and place one on a serving plate. Sprinkle with one-fourth of the remaining rum and spread with one of the three custard mixtures. Repeat with the second and third layers. Cover with the fourth layer of cake and sprinkle it with the remaining rum. Chill overnight in the refrigerator.