Cook millet in the vegetable broth for 5 minutes and drain. Rinse cabbage, pluck off 8 leaves and blanch for about 3 minutes in boiling salted water. Rinse and slice mushrooms. Mix the millet with the mushrooms, cheese and egg. Season with salt, pepper and cayenne pepper. Cut the thick stems off the cabbage leaves. Spread the millet mixture on the leaves, roll up and place, seam-side-down, in an ovenproof greased baking dish. Cover with aluminum foil and cook in preheated oven at 200°C (fan: 180°C, gas mark 3) (approximately 400°F) for 30 minutes.
Heat sour cream. Peel the potatoes, finely grate and boil in the sour cream. Rinse chives, cut into small rings, add to the sour cream and season with salt and pepper. Arrange cabbage rolls on plates, topped with the potato-chive sauce.