01 - Whisk lemon zest, lemon juice, sugar, whole eggs, egg yolk, and salt in a heatproof bowl until smooth and fully blended.
02 - Place bowl over gently simmering water, whisk constantly until mixture thickens to coat the back of a spoon, about 8 to 10 minutes; avoid boiling.
03 - Remove from heat and whisk in butter cubes gradually until completely melted and curd is glossy and uniform.
04 - Pass curd through fine-mesh sieve into a clean bowl to remove any cooked egg solids and zest for a smooth texture.
05 - Cover surface with plastic wrap to prevent skin formation, cool to room temperature, then refrigerate for at least 2 hours to thicken.
06 - Apply as filling for tarts, cakes, pastries, or spread on scones and toast.