In a small sauce pan, whisk together the sugar and the yolks.
Add the lemon juice, slowly, and cook over medium heat, stirring constantly until it thickens and coats the spoon.
Remove from the heat and whisk in the butter and the zest. Set aside to cool to room temperature.
Fold in whipped cream. Refrigerate until needed.
Notes
Lemon curd (before whipped cream is added) may be refrigerated for up to a week. Place in a covered container, with a layer of plastic wrap flush with surface of curd.