01 - In a heatproof bowl, vigorously whisk the eggs and granulated sugar until the mixture is completely blended and pale.
02 - Stir in the freshly grated lemon zest and strained lemon juice until fully combined.
03 - Position the bowl over a saucepan of lightly simmering water, ensuring the bottom of the bowl does not touch the water.
04 - Constantly whisk the mixture for 8 to 10 minutes, or until it thickens and can coat the back of a spoon.
05 - Remove from heat and immediately add cubed unsalted butter, whisking until fully melted and the curd is smooth.
06 - Strain the lemon curd through a fine-mesh sieve into a clean bowl to eliminate any bits of zest or coagulated egg.
07 - Let the mixture cool briefly, then transfer into sterilized jars. Refrigerate for up to two weeks.