Cream the butter. Mix the flour and sugar together, add gradually to the creamed fat and beat until smooth. Separate the egg and add the well-beaten yolk to the mixture with 50 g water and a few drops of vanilla essence.
Cook in a small saucepan over a very low heat, stirring all the time (if preferred, use a double pan or a basin over hot water; the mixture must not boil or it will curdle). Stir until sauce thickens.
Remove from heat, stir in the sherry. Just before serving, fold in the stiffly-beaten egg white.