Wash and pat dry lemons with a clean kitchen towel. Grate the zest and squeeze out the juice. In the meantime, prepare a pot in which you'll put water to a boil.
Crack the eggs into a bowl and mix them with an electric mixer for a minute, just to make sure that you won't be left with any egg white strains.
Put the lemon zest, lemon juice, and sugar into a heatproof glass bowl, stir it, and let it sit over a pot with slightly simmering water (bain-marie). Slowly start stirring. When sugar dissolves, add eggs and keep stirring constantly.
When the eggs have been incorporated well with melted sugar and juice, add the cubed butter stirring constantly. Make sure that the water is slightly simmering, not boiling. The curd is done after it becomes thick enough to cover the back of the spoon. Remove the lemon curd from the heat and set it aside.
If you use the curd the same day, put the plastic wrap on the bowl in which you've put the curd because you want to avoid creating a crust on the surface. If you won't use it immediately, pour a still-hot lemon curd into sterilized jars, close them, and let them cool. When cooled, place the jars in a refrigerator where you can keep them unopened for a maximum of 3 weeks. When opened, eat it within 7 days.