In a small blowl, blend water smoothly with cornflour. Set aside.
In a large pan, combine stock, carrot and soy sauce. Bring to the boil, then simmer over a medium heat for 5 minutes.
Add prawns, cabbage, spring onions and water chestnuts to stock mixture, cook for 4 minutes.
Stir in corflour mixture, snow peas and sesame oil. Cook, stirrign continuosly, until soup boils and thickens slightly. Ladle soup into 4 serving bowls. Serve.