1. Spread the cooked sushi rice evenly on a sushi mat.
2. Place cucumber at the bottom of the rice.
3. Roll up the sushi tightly using the sushi mat.
4. Coat the outside of the rice roll with black and white sesame seeds.
5. Cut the sushi roll into slices of desired thickness.
6. Blanch the prawns in boiling water for one minute, then drain and let them cool.
7. Place a dab of wasabi on the bottom of each prawn and place one on top of each sushi slice.
8. Serve immediately.