01 - In a heatproof bowl, thoroughly whisk together eggs and granulated sugar until the mixture is smooth and homogenous.
02 - Add lemon zest and freshly squeezed lemon juice, stirring until evenly integrated.
03 - Set the bowl over a saucepan of gently simmering water, ensuring the bottom of the bowl does not touch the water.
04 - Whisk the mixture constantly, allowing it to cook gradually until thickened and capable of coating the back of a spoon, approximately 8 to 10 minutes.
05 - Remove from heat immediately and whisk in cubed butter until completely melted and the curd reaches a silky consistency.
06 - Pour the curd through a fine-mesh sieve into a clean bowl to eliminate any residual zest or cooked egg fragments.
07 - Allow the lemon curd to cool fully, then transfer to sterilized jars. Store under refrigeration for up to two weeks.