Melt the butter in the pan and add the onion and garlic and soften.
Place the cumin seeds in a pan and dry fry then for 2 to 3 minutes until they release their aroma.
Add the cumin seeds to the soup.
Add the potato and water with the chicken stock cube and stir.
Simmer for 10 minutes or until the sweet potato is soft.
Transfer to a blender and purée.
Serve with a dollop of yoghurt