Wash potatoes thoroughly. Cut a thin slice from top of each potato. Carefully scoop out potato without harming skin, leaving a 1/2-inch shell. (If necessary, to make potatoes sit flat, cut a thin slice from the bottom.) Place shells in a large bowl. Cover with water until needed. Finely chop scooped-out potato.
In a large skillet cook chopped potato, onion, and mushrooms in hot butter or margarine until onion is tender. Stir in flour, the 1 teaspoon parsley, salt, pepper, and paprika. Cook until golden. Add milk; cook until slightly thickened. Stir into beaten egg yolk.
Drain potato shells and fill with cooked potato mixture. Place in shallow baking pan. Bake potatoes, uncovered, in a 450 degree F oven about 35 minutes or until potato shells are tender. If desired, garnish with additional parsley. Makes 6 servings.