Combine the lemon juice and zest in a small saucepan over medium-high heat, stirring occasionally.
Meanwhile, in a large heat-proof bowl, whisk together the egg and sugar.
Once the mixture in the saucepan comes to a boil remove from the heat and slowly pour it into the egg mixture, whisking continuously to temper the eggs.
Pour the mixture back into the saucepan and heat over low heat, whisking continuously until the mixture thickens to the point where it can coat the back of a spoon, about 10 minutes, then remove from heat and allow to cool slightly for 10 minutes.
Stir in the butter, one cube at a time, until smooth.
Store in an airtight container in the fridge for up to one week.