01 - In a heatproof bowl, whisk together the eggs and granulated sugar until the mixture is smooth and thoroughly blended.
02 - Add the freshly grated lemon zest and lemon juice to the egg and sugar mixture, stirring to combine.
03 - Place the bowl over a saucepan of gently simmering water, ensuring the bottom of the bowl does not touch the water.
04 - Whisk continuously over medium heat until the mixture thickens and coats the back of a spoon, about 8 to 10 minutes.
05 - Remove the bowl from heat and immediately whisk in the cubed butter until completely melted and the mixture is smooth.
06 - Pour the lemon curd through a fine-mesh sieve into a clean bowl to remove any solids or zest.
07 - Let the curd cool to room temperature, then transfer to sterilized jars and refrigerate for up to two weeks.