Cook blackberries in water and salt until berries are soft. Rub through a sieve and add sugar to pulp; return to range and cook until mixture thickens, stirring occasionally. Sift flour with baking powder and salt, work in Cottolene with tips of fingers, and mix to a soft dough with yolk of egg mixed with one-half cup of milk. Turn onto a floured board, knead slightly and roll out in a rectangular sheet one-fourth inch thick. Divide this into four pieces, longer than wide. Spread each with the blackberry sauce and roll up like jelly roll; wet the edges, press lightly to prevent unrolling. Lay on buttered sheet and brush tops with white of egg, sprinkle with sugar and a few grains cloves. Bake twenty-five minutes in a hot oven. Serve hot with remaining sauce kept hot over hot water or with Creamy Sauce.