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1
Melt the butter in the top of a double boiler over simmering water.
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2
Beat the eggs and sugar in a bowl until light and foamy.
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3
Add the mixture to the melted butter.
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4
Cook gently, stirring constantly, until the mixture becomes a custard, about 8 minutes.
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5
Do not overcook or the eggs will scramble.
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6
Remove the custard from the heat and stir in the lime juice and grated zest.
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7
Cool to room temperature.
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8
This step is unorthodox but crucial.
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9
Using an electric mixer, whip the chilled cream until very stiff, almost, but not quite, to the point where it would become butter.
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10
Stir the lime custard into the whipped cream until just incorporated.
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11
Pour into 8 individual serving glasses or a serving bowl.
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12
Chill for at least 4 hours.
