01 - In a medium heatproof bowl, whisk together lemon zest, lemon juice, sugar, eggs, egg yolk, and salt until smooth.
02 - Place the bowl over a saucepan of gently simmering water, ensuring the bowl does not touch the water (double boiler method).
03 - Add butter cubes and cook, whisking constantly, until the mixture thickens and coats the back of a spoon, about 10 to 15 minutes. Avoid boiling.
04 - Remove from heat and strain the curd through a fine-mesh sieve into a clean bowl to remove zest and cooked egg bits. Let cool to room temperature.
05 - Transfer to sterilized jars and refrigerate. The curd will thicken further as it cools.