In a double boiler, over low heat combine the sugar, lemon zest and eggs. Whisking thoroughly, add the lemon juice and continue to whisk the mixture. As the mixture warms, add the butter chunks. With a thermometer check the temperature, you want it to be around 170 degrees Fahrenheit (or stir until thickened if you don't have a thermometer). Once thickened, take it off the heat and stir for a couple of minutes. Store in the refrigerator.