In eleven-inch iron skillet, melt together butter and brown sugar. Add pineapple rings and cook until bubbly. Sift flour, baking powder and salt into bowl. Beat egg yolks; add sugar, a little at a time, and vanilla. Stir in three-eighths cup hot water, and combine with dry ingredients. Fold in stiffly beaten egg whites. Pour batter over pineapple rings. Bake in skillet at 325° for fifty minutes. Cool for ten minutes, invert onto platter. Serve with whipped cream or light cream.