In a glass bowl, Rub zest well into sugar to release the oils.
Add eggs and whisk till lighter and fluffy.
Whisk in the citrus juice the place bowl onto a pan of simmering water.
Stir till it thickens enough that your finger leaves a trail on the spatula.
Remove from the heat and whisk in the cubes of butter till amalgamated.
Pour into a jar and keep in the fridge till cooled & thickened, then it's ready to use!
