Preheat oven at 475. Prepare a 1 1/2 quart baking dish with cooking spray. Boil potatoes until tender. Mash with margarine and 1/4 cup yogurt. Season with salt and pepper. Whip the remaining 1/4 cup yogurt until stiff. Spread mashed potatoes in a baking dish. Sprinkle with grated Parmesan cheese. Bake until the cheese melts and the top is golden brown, about 20 minutes.