Beat the butter and sugar to a cream; add the well-beaten egg, and then the corn starch, which you will have dissolved in two tablespoonfuls of water. Combine with butter and sugar. When well blended, pour in slowly the boiling water, stirring all the time, and set over the fire till it thickens; stir about three minutes. Take from the fire and add the juice and rind of a grated Lemon. This Sauce is much improved by adding the white of an egg, beaten to a stiff froth, after you have taken it from the fire.