1. Firstly get the chilli roasting. If you have a gas hob or BBQ then place a wire rack over the flames and pop the chilli on there, turning each time the side goes black and charred. If roasting in an oven then pop in foil and roast in a moderate oven for 30 mins. I like to add a black cardamom pod in there too, but don't worry if you don't have any.
2. In a large pan gently heat the olive oil and butter.
3. Add the cumin seeds and black pepper and sizzle for 30 seconds before stirring in the leeks.
4. After 1 minute add the carrots and vegetable stock, adding more boiling water if you think you need to.
5. Stir through half the coriander, bring to a simmer and allow to cook for 15 minutes, or until the carrots are soft.
6. Check the chilli. If it is blackened all over then place in a bowl covered with clingfilm. When hot enough to handle carefully peel off the skin and discard.
7. Roughly chop the chilli and stir half through the soup. Save the rest for garnish.
8. Blitz the soup in a blender until almost smooth and pour back into the pan to reheat.
9. Taste and adjust the seasoning.
10. Serve with a dollop of yoghurt and a final sprinkling of coriander and roasted chilli.