Melt butter in stockpot over medium heat. Add carrots, onion, potato, and garlic and ginger. Cover and cook until vegetables are tender, about 20 minutes. Add Kosher salt, black pepper, and ground cumin and then stir for about 1 minute. Add broth and milk. Bring to a low boil, about 15 minutes. Remove from heat to cool, about 15 minutes.
When the soup has cooled. Transfer to blender, puree in two or three batches. Return to stove and reheat. Serve hot adding a dollop of yogurt to each bowl.