Finely grate the zest from the lemon and set aside.
Cut the lemon in half and squeeze out the juice.
In a bowl, combine the eggs, erythritol, lemon juice, and lemon zest and mix well.
Pour the mixture into a double boiler and cook while stirring.
Once the mixture thickens, remove from heat and add the butter, stirring until melted.
Strain through a sieve for a smooth texture and allow it to cool.