01 - Wash and dry the lemons. Finely zest them, then juice to obtain approximately ½ cup of lemon juice.
02 - In a medium heatproof bowl, whisk together the sugar, whole eggs, and egg yolk until the mixture is smooth and homogeneous.
03 - Incorporate the lemon zest and freshly squeezed lemon juice into the egg mixture, stirring to combine evenly.
04 - Place the bowl over a saucepan of gently simmering water, ensuring the bowl's bottom does not touch the water. Whisk continuously for 8 to 12 minutes until the curd thickens sufficiently to coat the back of a spoon.
05 - Remove the bowl from heat and immediately whisk in the cubed butter until fully melted and the curd achieves a smooth consistency.
06 - Pass the lemon curd through a fine-mesh sieve into a clean bowl to eliminate zest fragments and any coagulated egg bits.
07 - Allow the curd to cool to room temperature before transferring it into sterilized jars. Refrigerate and consume within two weeks.