01 - In a medium heatproof bowl, whisk together the eggs, egg yolk, sugar, lemon zest, and lemon juice until fully blended and smooth.
02 - Place the bowl over a saucepan filled with 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 press the curd through a fine-mesh sieve into a clean bowl to remove any zest bits or cooked egg strands.
05 - Cover the surface directly with plastic wrap 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.