01 - Wash lemons thoroughly, zest them using a fine grater, then extract the juice, reserving both.
02 - In a medium heatproof mixing bowl, whisk together the granulated sugar, whole eggs, and egg yolk until smooth and homogenous.
03 - Stir in the lemon zest and juice into the egg and sugar mixture evenly.
04 - Place the mixing bowl over a saucepan of gently simmering water, ensuring the bowl does not touch the water surface.
05 - Add the butter cubes gradually, whisking constantly to prevent curdling. Continue cooking until the mixture thickens and coats the back of a spoon, approximately 8 to 10 minutes. Avoid boiling.
06 - Remove from heat and strain the curd through a fine-mesh sieve into a clean container to remove zest particles and cooked egg solids.
07 - Allow to cool to room temperature before transferring to a jar. Refrigerate until chilled and thickened.