1. Melt butter in large, heavy casserole over low heat.
2. Add the onion and cook until soft and lightly browned, about 7 minutes.
3. Stir in dark brown sugar, curry powder, coriander, cardamom, grated nutmeg, and ginger root.
4. Cook one minute longer. Stir in carrots, potatoes, and chicken stock.
5. Allow it to boil for a few minutes.
6. Lower heat and simmer 30 minutes or until carrots are very tender.
7. Strain the soup, reserving the liquid and solid separately.
8. Place solids in a container of food processor or blender.
9. Cover and whirl until pureed. Whisk puree into liquid.
10. Return to casserole (the soup may be made ahead to this point and frozen).
11. Add cream, simmer 10 minutes.
12. Season with salt and pepper to taste.
13. Serve hot, garnish with coriander or mint leaves.