Combine sugar and cornstarch in heavy saucepan add water and stir until heated, set aside.
Beat egg yolks and add lemon juice, stir in small amount of sugar mixture to warm yolks. Combine yolk and sugar mixtures, add butter and return to heat.
Bring to a rolling boil, stirring constantly until thickened. Mixture will resemble pudding. Cool slightly and spread on cooled cake. Makes enough to frost and fill one 8 inch four layer cake.
