Marinate duck with 2 tablespoons light soy sauce and 1 tablespoon thick soy sauce. Pre-fry the duck and soaked chestnuts for a while. Dish out and rinse under running water (to remove the excess oil).
Heat oil and sesame oil in wok. Add the herbs and stir-fry well. Mix in duck, chestnuts and seasoning. Dish out the contents onto a piece of glass paper. Thicken the gravy with the cornflour mixture and pour over the duck. Tie up and wrap in aluminium foil into a packet. Pressure cook for 40 minutes.
Remove the package and transfer to a serving dish. Serve immediately.