In the top pan of a double boiler, whisk the eggs, sugar and key lime juice.
Continue to whisk over medium heat. When the mixture starts to thicken, add a slice of butter, one at a time until you've added it all. Don't stop whisking. Keep whisking until the butter has completely melted. The mixture should be pretty thick.
Pour the softened gelatin to the lime mixture.
Remove from heat. Using a fine mesh strainer, strain the mixture to ensure you don't have any clumps or egg pieces left.
Stir the lime zest. Allow to cool in the refrigerator until ready to use.