In 2-quart saucepan, sprinkle gelatin on cold water to soften. In medium bowl, beat 1/2 cup sugar and 2 eggs with electric mixer on high speed until thick and lemon colored; stir into gelatin mixture. Cook about 15 minutes or just until boiling over low heat, stirring constantly. Remove from heat. Stir in lemon peel, lemon juice and 2 tablespoons butter.