Heat the oven to 200C (180C fan)/390F/gas 6.
Cut the croissants in half, then tear them into 2cm chunks.
Put the butter in a large heatproof bowl, melt in the microwave, then stir in the croissant pieces and sugar, until evenly coated.
Scatter the croissant pieces over a large baking sheet lined with greaseproof paper, and bake for 10-14 minutes until well browned and caramelised.
Remove, leave to cool for a couple of minutes, then tip into a food processor and add the salt.
Blitz to a coarse powder, then add the chocolate and blitz again to a stiff paste.
Finally, mix in the oil, then decant into a sterilised jar to continue cooling.
Once sealed, the croissant butter will keep in a cool cupboard for up to a month or in the fridge for three months.
