Pour the lemon juice, zest, butter and Erythritol into a heatproof bowl.

Place the bowl over a pan of simmering water.

Stir the mixture until all the butter has melted.

Whisk the eggs and yolk and pour into the lemon mixture, continue to heat until the mixture thickens enough to coat the back of a spoon.

Pour the mixture into a sterilized jar and allow it to cool and set.

Store in the regrigerator.