Whisk together the granulated sugar, eggs and lime juice in a stainless steel or heat proof bowl. Place over the top of a pan of simmering water and cook, whisking constantly so the egg doesn't curdle, until the mixture thickens to the consistency of sour cream, about 10 minutes. 3/4 cup granulated sugar,3 large eggs,1/2 cup fresh lime or key lime juice

Remove from heat and pour the curd through a fine mesh strainer placed over a medium bowl, using a spoon to help it through. Add the butter, and stir until completely melted. Stir in the lemon zest. Let the curd stand at room temperature. The curd will thicken as it cools. 3 tablespoons unsalted butter,1 1/2 tablespoons lime zest

Once the curd has cooled, place plastic wrap over the surface of the curd to keep a skin from forming, and store in the refrigerator.