Place the juice in a small saucepan. Over medium-high heat, reduce the liquid by half.
Remove the pan from the heat and stir in the sugar and butter. Stir in the egg until well combined.
Return the pan to the burner and cook on medium-low heat for 3 to 5 minutes or until bubbles just begin to form.
Remove the pan from the heat and stir in the citrus zest. Place the pan in a bowl of ice and stir the mixture until it is cold.
In another bowl, whip the cream until stiff. Fold the citrus mixture thoroughly into the cream.
Make sure that you remove the pan from the heat source before you stir in the egg or it will scramble right before your eyes and the recipe will be ruined.