01 - In a heatproof bowl, thoroughly whisk together eggs and granulated sugar until the mixture is uniform.
02 - Add lemon zest and freshly squeezed lemon juice, continuing to whisk until fully blended.
03 - Place the bowl over a saucepan of gently simmering water, ensuring the bottom of the bowl does not touch the water.
04 - Whisk constantly while cooking until the mixture thickens and coats the back of a spoon, approximately 8 to 10 minutes.
05 - Remove the bowl from heat and immediately whisk in cubed butter until completely melted and the curd is smooth.
06 - Pour the curd through a fine-mesh sieve into a clean bowl to remove zest and any cooked egg solids.
07 - Allow the lemon curd to cool, then transfer to sterilized jars. Refrigerate and consume within 2 weeks.