Pour water into a Copper-Core 2 qt. Sauce Pan and bring to a simmer over medium-high heat. Put the porcelain double boiler insert into the sauce pan and add the butter. Once the butter has melted, add the sugar, whisking to dissolve it, then add the zest and juice, and heat until the mixture is warm, but not simmering.
Whisk in the eggs and continue to whisk for 4 to 5 minutes or until the curd has thickened. You should be able to see the trail of the whisk through the curd. Strain the curd through a fine-mesh strainer into a bowl. Let cool for a few minutes, then cover with cling film pressed directly against the curd and refrigerate until ready to use. The curd can be refrigerated for up to 2 weeks.
Makes 2 cups.