1. Make lemon curd: In a medium saucepan, whisk together butter, 1 cup sugar, eggs, yolks, and lemon and lime juices (mixture may appear curdled). Place over low heat; cook, stirring, until smooth, 4 to 5 minutes. Raise heat to medium; cook, stirring constantly, until thick enough to coat back of spoon, 4 to 8 minutes (do not boil).
2. Remove pan from heat; stir in lemon zest. Transfer mixture to a bowl; cover with plastic wrap, and chill at least 1 hour.
3. In a mixing bowl, beat cream and remaining 1/3 cup sugar to soft peaks. Whisk lemon curd to loosen, then gently fold in whipped cream. Spoon into six glasses. Cover; chill at least 2 hours and up to 3 days. Just before serving, garnish with zest.

Yield: 6 servings