Pre-heat the oven to 120°C (fan)/140°C/gas mark 1. Lightly grease two baking trays, or line with greaseproof paper.
Wash the kale well then remove the leaves from the stem. You can cut them off with a sharp knife or hold the centre stem and tear the leaves downwards to remove them.
Dry your kale leaves using a salad spinner or by patting them dry with kitchen towel or a clean tea towel. The less liquid on them, the crispier your kale crisps will be.
Put the leaves into a mixing bowl and drizzle over the olive oil, ground cumin and salt if using. Massage the oil and spices into the leaves using your hands and spread thinly over the two baking trays.
Place in the oven for 10 minutes then turn the trays and turn over any kale chips that look in danger of burning. Return to the oven for another 10 minutes.
Leave for 5 minutes on the tray to cool a little as this will help them to crisp up further.
Allow them to cool completely then store your kale chips in an airtight container for several days.