1 In a saucepan, melt butter over low heat.
2 Beat in egg yolks, one at a time, using a wire whisk.
3 Remove from the heat.
4 Stir in the juice from lemons and limes, 4 packets sugar substitute, and lemon rind.
5 Beat well.
6 Allow to cool.
7 In a bowl, beat egg whites with 1 packet sugar substitute and vanilla extract.
8 Gradually fold into the chilled mixture.
9 Refrigerate for at least 2 hours.