01 - In a heatproof bowl, thoroughly whisk the eggs with granulated sugar until the mixture is smooth and well combined.
02 - Stir the lemon zest and freshly squeezed juice into the egg-sugar mixture until evenly integrated.
03 - Set the bowl over a saucepan of gently simmering water, ensuring the base of the bowl does not touch the water.
04 - Whisk the mixture constantly 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 the cubed butter until fully melted and the consistency is velvety smooth.
06 - Pour the curd through a fine-mesh sieve into a clean bowl to filter out any zest or cooked egg solids.
07 - Allow the lemon curd to cool before transferring into sterilized jars. Refrigerate for up to two weeks.