Prepare a double boiler: Fill a medium saucepan with water and bring to a simmer. Place a heatproof bowl over the saucepan, ensuring the bottom of the bowl doesn't touch the water.
Combine Ingredients: In the bowl, whisk together the egg yolks, whole egg, sugar, lemon juice, lemon zest, and salt until well combined.
Cook the Mixture: Continuously whisk the mixture over the simmering water until it thickens and coats the back of a spoon, about 10-15 minutes. Make sure not to let it boil.
Add Butter: Remove the bowl from the heat and whisk in the butter until fully melted and smooth.
Strain (Optional): For an ultra-smooth texture, strain the lemon curd through a fine-mesh sieve into a clean bowl.
Cool and Store: Let the lemon curd cool to room temperature, then transfer it to an airtight container. Refrigerate for up to one week.