Melt butter in top of a double boiler over boiling water. Gradually stir in sugar and lemon rind and juice. Gradually stir 1 tablespoon hot mixture into beaten egg; add to remaining hot mixture, stirring constantly. Cook over medium heat, stirring until sauce thickens.
Pour lemon sauce into hot sterilized jars. Cover with metal lids, and screw bands tight. Serve warm or chilled over ice cream, pound cake, or steamed pudding. Store in refrigerator until ready to serve.