Sift together the flour, baking powder, salt and 1 tablespoon of the sugar. With a pastry blender, work in the butter. Stir in the milk to make a dough. Roll out the dough on a lightly floured board and cut into 5 cm squares. Heat the grape juice, grapes and remaining sugar in a heavy kettle until boiling. Drop in the dumplings. Cover closely and steam 15 to 20 minutes or until tender. Serve hot.