01 - Zest the lemons using a microplane, then juice them until you have about ½ cup of fresh lemon juice. Set both aside.
02 - In a medium heatproof bowl, whisk together the granulated sugar, whole eggs, and egg yolks until completely smooth.
03 - Stir in the lemon zest and fresh lemon juice until evenly combined.
04 - Place the bowl over a saucepan of gently simmering water, ensuring the bottom of the bowl does not touch the water surface.
05 - Add the butter cubes to the bowl. Cook the mixture, 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 from heat and strain the lemon curd through a fine-mesh sieve into a clean bowl to remove the zest and any bits of cooked egg.
07 - Cool to room temperature, then transfer to sterilized jars. Refrigerate for up to 2 weeks.