Place sugar and orange zest in the food processor and pulse several times. This helps to colour the sugar and releases the orange oil, intensifying the flavour. Place sugar and zest in a small, heavy-bottom saucepan.
Add juice, eggs, yolks and whisk all together.
Add butter and place over medium-low heat, stirring continuously.
Cook for approximately 8 minutes, stirring all the while, until mixture thickens and coats the back of a spoon.
Strain it (if you can be bothered) and place in sterilised jars. Refrigerate for up to two weeks (if it lasts that long).