01 - In a heatproof bowl, whisk together eggs and granulated sugar until fully blended.
02 - Whisk in lemon zest and freshly squeezed lemon juice until the mixture is smooth.
03 - Place the bowl over a saucepan with gently simmering water, ensuring the bowl does not touch the water.
04 - Whisk the mixture constantly over gentle heat until it thickens and coats the back of a spoon, approximately 8 to 10 minutes.
05 - Remove the bowl from heat and immediately whisk in cubed unsalted butter until fully incorporated and the curd is emulsified.
06 - Pour the lemon curd through a fine-mesh sieve into a clean bowl to remove zest and any cooked egg.
07 - Let the curd cool, then transfer into sterilized jars. Seal and refrigerate; use within two weeks.