01 - In a medium heatproof bowl, whisk together the eggs, egg yolk, sugar, lemon zest, and lemon juice until smooth and well blended.
02 - Fill a saucepan with about 1 inch of water and bring to a gentle simmer. Place the heatproof bowl over the saucepan, making sure the bottom of the bowl does not touch the water.
03 - Add the cubed butter to the bowl and cook, whisking constantly, until the mixture thickens enough to coat the back of a spoon, about 10 to 15 minutes.
04 - Remove the bowl from heat and pour the curd through a fine-mesh sieve into a clean bowl, pressing through with a spatula to remove any solids.
05 - Press plastic wrap directly onto the surface of the curd to prevent a skin from forming. Allow to cool to room temperature, then refrigerate for at least 2 hours before serving for the best texture.