01 - In a heatproof bowl, vigorously whisk eggs with granulated sugar until mixture is thoroughly blended and smooth.
02 - Add the lemon zest and freshly squeezed lemon juice to the egg mixture, stirring until fully combined.
03 - Place the bowl over a saucepan of gently simmering water, ensuring the base of the bowl does not touch the water.
04 - Continuously whisk the mixture over gentle heat until it thickens and coats the back of a spoon, approximately 8 to 10 minutes.
05 - Remove from heat and immediately whisk in cubed unsalted butter until completely melted, yielding a glossy and smooth mixture.
06 - Strain the lemon curd through a fine-mesh sieve into a clean bowl to remove any solids.
07 - Let the lemon curd cool to room temperature, then transfer to sterilized jars and refrigerate for up to 2 weeks.