01 - Zest the lemons using a microplane, then juice them until you have about ½ cup of fresh lemon juice. Set both the zest and juice aside.
02 - In a medium heatproof bowl, whisk together the granulated sugar, whole eggs, and egg yolks until completely smooth and well blended.
03 - Stir the lemon zest and fresh lemon juice into the egg-sugar mixture until evenly incorporated.
04 - Place the bowl over a saucepan of gently simmering water, ensuring the bottom of the bowl does not touch the surface of the water.
05 - Add the cubed butter to the bowl. Cook the mixture over the double boiler, whisking constantly, until thickened and creamy, about 7 to 10 minutes. The curd is ready when it coats the back of a spoon.
06 - Remove the bowl from heat. Strain the lemon curd through a fine-mesh sieve into a clean bowl to remove the zest and any bits of cooked egg.
07 - Allow the curd to cool to room temperature, then transfer to sterilized jars. Refrigerate for up to 2 weeks.