01 - In a heatproof bowl, vigorously whisk the eggs and granulated sugar until fully blended and slightly lightened in color.
02 - Stir in lemon zest and fresh lemon juice until fully integrated.
03 - Place the bowl securely over a saucepan containing gently simmering water, ensuring the bowl does not touch the water underneath.
04 - Continuously whisk the mixture until it thickens enough to coat the back of a spoon, approximately 8 to 10 minutes.
05 - Remove the bowl from heat and immediately whisk in cubed unsalted butter until completely melted and curd is silky smooth.
06 - Pour the lemon curd through a fine-mesh sieve into a clean bowl to eliminate any zest pieces or cooked egg fragments.
07 - Allow the curd to cool before transferring to sterilized jars; refrigerate for up to 2 weeks.