Pre-heat oven to 350. In a 9" (23cm) round cake pan or springform pan, melt the 2 oz. of butter with the brown sugar over low heat, stirring constantly until the sugar is completely dissolved. Pour the cranberries onto the brown sugar and butter mixture. Set aside. In a bowl or mixer, cream the 4 oz. of butter with the granulated sugar until smooth. Stir in the yolks and vanilla. In a large bowl combine the flour, baking powder and salt. Alternately mix the milk and the dry ingredients. Stir only until mixed. Beat the egg whites to stiff peaks, then fold into the cake mixture. Pour cake mixture over cranberries. Bake at 350 for 50 or 60 minutes or until the cake is done.