Lemon Sauce

Stir 1 tablespoonful butter with 4 tablespoonfuls powdered sugar to a cream; add by degrees 1 beaten egg, the juice and grated rind of ½ lemon, a little nutmeg and 4 tablespoonfuls boiling water; beat the sauce thoroughly for 5 minutes; put in a tin pail and set in saucepan of hot water; stir constantly until very hot, but do not allow it to boil.