Scald milk in top of double boiler. Mix together sugar, corn starch & salt; pour hot milk over this mixture; stir, return to double boiler, cook over boiling water until thick. Cover & continue cooking for 15 minutes, stirring occasionally. Beat egg yolk lightly and pour hot mixture over it, mix well. Return to double boiler and cook for 3 minutes. Remove from heat, add flavour and butter, mix well. Cool before using. Makes enough to fill 9 inch layer cake.