Prepare the Double Boiler: Place a heatproof bowl over a medium saucepan filled with gently simmering water, ensuring the bowl does not touch the water.

Whisk Together Ingredients: In the prepared heatproof bowl, whisk together eggs, egg yolks, sugar, lemon juice, zest, and salt until smooth.

Cook the Mixture: Cook the mixture over the simmering water for about 8 to 12 minutes, whisking constantly until it thickens.

Incorporate the Butter: Remove from heat and gradually whisk in the butter until completely melted and smooth.

Strain for Smoothness: Strain the curd through a fine-mesh sieve into a clean bowl to remove zest and bits of cooked egg.

Cool the Curd: Press plastic wrap directly onto the surface of the warm curd and allow it to cool at room temperature for 30 minutes.

Chill Before Serving: Transfer the curd to the refrigerator and chill for at least 2 hours before using.