Step 1: In a small, heavy-bottomed saucepan, whisk together the egg yolks, granulated sugar, lemon zest, and salt until the mixture is well blended and smooth.
Step 2: Slowly whisk in the fresh lemon juice until fully incorporated into the egg mixture.
Step 3: Place the saucepan over low heat. Cook the mixture, stirring constantly with a whisk or a silicone spatula, making sure to scrape the bottom and sides of the pan.
Step 4: Continue cooking for about 8 to 12 minutes, or until the mixture thickens significantly and can coat the back of a spoon (it should reach approximately 170°F/77°C). Do not let it boil.
Step 5: Remove the pan from the heat and immediately whisk in the cold, cubed butter one piece at a time until the butter is completely melted and the curd is glossy.
Step 6: Optional: For an ultra-smooth texture, pour the lemon curd through a fine-mesh sieve into a bowl to remove the zest.
Step 7: Press a piece of plastic wrap directly onto the surface of the curd to prevent a skin from forming, and refrigerate until completely chilled and set.