01 - In a heatproof bowl, whisk together eggs and granulated sugar until the mixture is homogenous.
02 - Mix in the freshly grated lemon zest and lemon juice, stirring to blend.
03 - Place the bowl over a saucepan with gently simmering water, ensuring the bottom of the bowl does not touch the surface of the water.
04 - Whisk the mixture consistently over gentle heat until it thickens enough to coat 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 curd becomes smooth.
06 - Pour the lemon curd through a fine-mesh sieve into a clean bowl to remove zest and any coagulated egg.
07 - Allow the curd to cool to room temperature, then transfer to sterilized jars. Seal and store in the refrigerator for up to two weeks.