Blanch the beans in a pan of boiling water for a few minutes then refresh in iced water. Drain and then cut into short lengths.
Toast the sesame seeds in a dry frying pan, taking care not to burn them. Grind them using a pestle and mortar while they are still hot. When they are half-paste like, add the sugar, oil and soy and continue to grind until you have a smooth dressing.
Mix the beans with the dressing and serve.