To make the lime curd, heat the sugar, lime juice, lime zest, and butter in the top of a double boiler until the butter is just melted. Whisk in the eggs, and bring the heat down to medium-low. Continue cooking and whisking the mixture in the top of a double boiler until it thickens considerably, about 20-30 minutes, Remove from heat and allow to cool before placing in a clean mason jar and sealing with a cap. Keep refrigerated.