In a double boiler, add the lemon juice, yolks, eggs, and sugar. It will thicken as it heats up. When the curd is thick enough to coat the back of a wooden spoon it is done. Mine took 25 minutes. Also, if you don't have a double boiler, put about an inch of water in a saucepan and let it simmer. Place a heat proof bowl on top of the saucepan. Make sure the bowl and the water aren't touching.
Strain the mixture with a fine mesh strainer and add butter and zest. Stir until the butter is fully incorporated.
Quickly place plastic wrap on the surface of the curd and immediately place in the fridge until completely cooled.