Start by washing and thoroughly drying the lemons. Zest all of the lemons to yield about 2 tablespoons of lemon zest. Then, juice the lemons to extract approximately ⅔ cup of fresh lemon juice. Make sure to strain the juice to remove any seeds or pulp.
In a heatproof bowl, preferably stainless steel or glass, whisk together the lemon zest, lemon juice, sugar, eggs, and a pinch of salt (if using) until well combined.
Set up a double boiler by placing a saucepan with a few inches of water on the stove over medium-high heat. Once the water starts simmering, reduce the heat to low and place the bowl with the lemon mixture on top, ensuring that the bottom of the bowl doesn't touch the simmering water. This indirect heat will gently cook the curd.
Using a silicone whisk, cook the lemon mixture while whisking constantly. The curd will thicken as it heats up, which should take about 10-15 minutes. Be patient and maintain a low simmer to avoid scrambling the eggs. The curd should be thick like a hollandaise sauce. (if the curd isn't thickening, turn the heat up while whisking constantly, watching carefully).
Once the curd has thickened and coats the back of a spoon, remove the bowl from the heat. Add the butter one piece at a time and whisk until it's fully melted and incorporated into the curd, resulting in a silky-smooth texture.
Cover the curd with a piece of plastic wrap (let it touch to prevent a skin from forming). Let the lemon curd cool for a few minutes in the bowl before transferring it to a clean jar or airtight container. Seal it and refrigerate for at least an hour to allow it to set and cool further.
The curd will last in the refrigerator for up to 10 days.