Trim the leaves off the coriander and finely chop the root and stems.
Heat the butter in a large saucepan over medium heat.
Add the corn kernels, capsicum, onion and chilli and stir to coat the vegetables in butter.
Cook covered, over low heat, stirring occasionally, for 10 minutes or until vegetables are soft.
Increase the heat to medium and add the coriander root and stem.
Cook, stirring for 30 seconds or until fragrant.
Sprinkle with the flour and stir a further minute.
Remove from the heat, and gradually add the vegetable stock, stirring together. Add 500 ml of water and return to the heat. Bring to the boil, then reduce heat and simmer, covered, for 30 minutes or until the vegetables are tender. Cool slightly.
Ladleabout 500 ml of the soup into a blender and puree until smooth.
Return the puree to the soup and add the yoghurt and gently heat until warm through. Season to taste.
Garnish with coriander leaves.