Make a double boiler: Place on the stove over medium heat a small saucepan filled with some water. Place another saucepan on top, so that it stays over the first one but it doesn’t touch the water. Place the beaten eggs, the lemon juice, lemon zest, sugar and salt in the top saucepan. Stir continuously with a silicone spatula.
When the water starts boiling, keep stirring to avoid the eggs to curdle into pieces. If needed, lower the heat but consider that the water has to boil so that the lemon curd temperature is high enough to kill any germs in the eggs (e.g. salmonella bacteria).
After stirring 10-15 minutes with the water underneath boiling, the lemon curd should be sufficiently thick. Remove the top pan from the heat and add the butter. Let it melt completely and stir gently until smooth.
Pass the lemon curd through a mesh sieve into a jar. This step is optional but it will help to remove any pieces of egg or lemon zest that might make the texture not smooth enough.
Store the lemon curd in the fridge, covered with a lid for up to 10 days.
Notes
The quantity of sugar varies according to your taste and the type of lemons you use. Some lemons are really sour while others (like Meyer lemons for instance) not as much.Make sure your lemons are not treated before using their zest. If you don’t have untreated lemons, you can buy lemon zest or just use lemon extract.