01 - In a heatproof bowl, whisk eggs together with granulated sugar until homogenous.
02 - Mix in lemon zest and fresh lemon juice until evenly blended.
03 - Place the heatproof bowl over a saucepan filled with gently simmering water, ensuring the base of the bowl does not contact the water.
04 - Whisk mixture continuously, maintaining gentle heat, until thickened and able to coat the back of a spoon, approximately 8 to 10 minutes.
05 - Remove bowl from heat and immediately whisk in unsalted butter cubes until fully melted and curd is smooth.
06 - Pass curd through a fine-mesh sieve into a clean bowl to remove zest and any cooked egg. Allow to cool before transferring to storage.
07 - Transfer to sterilized jars, seal, and refrigerate for up to two weeks.