In a dry small heavy skillet toast sesame seeds over moderate heat, shaking skillet, until golden and let cool. With a mandoline or other hand-held manual slicer or a sharp knife cut cucumbers into very thin slices. Sprinkle cucumbers with salt and drain in a colander 10 minutes.
In a bowl stir together vinegar and dashi granules until granules are dissolved. Add cucumber and oil, tossing to coat. Salad may be made 1 hour ahead and chilled, covered.
Serve salad at room temperature or chilled, sprinkled with sesame seeds.
