1. Boil the potatoes in their skins in the water for about 35 minutes or until tender.
2. Peel and while still hot, mash to a puree with the butter.
3. Stir in the baking powder and cream.
4. Turn into a well greased shallow casserole dish and bake in the oven at Gas mark 4, 350°F (180°C) for 25 minutes.
5. Sprinkle with the breadcrumbs and place under a hot grill to brown.