60ml lemon juice
2 teaspoons finely grated lemon rind
4 egg yolks
42 grams salted butter
Combine all in double boiler (a metal or glass bowl over a saucepan of water). Heat while whisking constantly until the mixture thickens and coats a spoon.
Pour into sterilised jars. Press a piece of greaseproof paper (or even a beeswax wrap) onto the top of the mixture to prevent a skin forming.
Refrigerate for up to 2 weeks. Freezes well.