01 - In a medium heatproof bowl, whisk together the eggs, egg yolk, granulated sugar, lemon zest, and fresh lemon juice until smooth and well blended.
02 - Place the bowl over a saucepan filled with about 1 inch of gently simmering water, making sure the bottom of the bowl does not touch the water surface.
03 - Add the cubed butter to the bowl and cook, whisking constantly, until the mixture thickens enough to coat the back of a spoon, approximately 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 ensure a silky, lump-free consistency.
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 optimal texture.