1. Melt the butter in the top of a double boiler over simmering water.

2. Beat the eggs and sugar in a bowl until light and foamy. Add the mixture to the melted butter. Cook gently, stirring constantly, until the mixture becomes a custard, about 8 minutes. Do not overcook or the eggs will scramble.

3. Remove the custard from the heat and stir in the lime juice and grated zest. Cool to room temperature.

4. This step is unorthodox but crucial. Using an electric mixer, whip the chilled cream until very stiff, almost, but not quite, to the point where it would become butter.

5. Stir the lime custard into the whipped cream until just incorporated. Pour into 8 individual serving glasses or a serving bowl. Chill for at least 4 hours.