01 - Wash and zest the lemons, then juice them.
02 - In a medium heatproof bowl, whisk together the granulated sugar, eggs, and egg yolk until smooth.
03 - Stir in the lemon zest and juice into the sugar and egg mixture.
04 - Place the bowl over a saucepan of gently simmering water, ensuring the bowl does not touch the water surface.
05 - Add the butter cubes; whisk constantly until the mixture thickens and coats the back of a spoon, about 8 to 10 minutes. Avoid boiling.
06 - Remove from heat and strain the curd through a fine-mesh sieve into a clean bowl, removing zest and any solids.
07 - Allow to cool to room temperature, then transfer to a jar and refrigerate to thicken further.