Core, peel and slice apples and place in bottom of 2-quart baking dish. Sift flour, baking powder, 1/2 teaspoon salt, cinnamon and 2/3 cup sugar. In separate bowl, combine milk, nuts, butter and 1 teaspoon vanilla. Stir into dry ingredients and spread on top of apples in baking dish. Mix together brown sugar, 1/4 cup granulated sugar, 1/2 teaspoon vanilla and 1/4 teaspoon salt. Sprinkle in casserole over batter. Pour boiling water over all. Do not stir. Bake, uncovered, at 350° for 30 minutes. Cover and bake 30 minutes more. Serve warm with whipped cream, if desired. Makes about 6 servings.