Add chicken stock, jam and soy sauce. Simmer for 15 minutes or until chicken is done.
Place the chicken breasts in a 23x33 cm (9x13 in) baking dish. Combine the apricot preserves, vinegar and sugar. Pour the preserve mixture over the chicken, cover and bake for 40 minutes. Remove cover and bake for 10 more minutes.
Remove chicken from pan. Add 1 tablespoon cornflour and 1 tablespoon water to sauce to thicken (equal amounts more of each if you like it thicker), and stir thoroughly. Return chicken to frying pan and turn to coat with sauce.