Toast the sesame seeds in a dry pan until golden, then crush 40g with the sugar using a pestle and mortar. Mix well with the soy sauce and dashi until you have a smooth dressing.
Cut the cucumber down the centre lengthways. Using a teaspoon, remove and discard the seeds. Cut into batons about 6cm long. Stir the dressing through, then sprinkle with the remaining sesame seeds.
Dashi, a fish- and kelp-based stock, is used in Japanese cooking for miso soup and noodle broths. Buy it at ocado.com or in Waitrose – or use fish stock in its place.