Squeeze the lemons and pour the juice in a deep saucepan. Add the zest of one lemon, sugar and 4 eggs and beat well.
Place the saucepan over medium-low heat and start cooking the mixture, stirring continuously.
This process takes 12-15 minutes.
It is ready when it starts to thicken and the curd covers the back of the spoon.
Remove the curd from the oven and pour it through a sieve in order to have a perfectly smooth curd, without the lemon zest or pulp, and also without some traces of cooked egg J
Cut the butter into cubes, and fold it in the lemon curd while it is still hot.