Mix starch and sugar in top of double boiler. Add juice and rind and mix until there are no lumps. Gradually add milk and water. Cook over simmering water, stirring constantly until thick. Cover, turn heat low, and cook for 10 min. Remove from heat. Beat egg yolk with a fork. Gradually add a little of the hot sauce into the yolk to heat it, beating constantly with a fork to keep it from curdling. Then pour mixture into double boiler and stir to blend. Cover and cool. Stir once or twice during first 10 min . to release steam.