Whisk together eggs, sugar, juice, and lemon zest. Set aside. Place water in the bottom of a double boiler or create one using a saucepan and glass bowl. When water begins to boil add butter to the top of the boiler (or glass bowl) When the butter begins to melt add the egg mixture. Whisk constantly until the mixture begins to thicken (about 10 minutes). Remove from heat and let cool for 5 minutes. Press plastic wrap inside of the bowl covering the top of the curd to prevent skin from forming. Let cool in the refrigerator completely. Store in air-tight jars for up to 1 week.