01 - In a medium heatproof bowl, whisk together the eggs, egg yolk, granulated sugar, lemon zest, and lemon juice until completely blended and smooth.
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 beneath.
03 - Add the cubed butter to the bowl and cook over the simmering water, whisking constantly, until the curd 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 cooked egg bits or zest fragments.
05 - Press plastic wrap directly against 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.