Rehydrate the shiitake mushrooms by soaking them in hot water (or cook them in Niban Dashi). Remove the stems and slice the mushrooms. Set aside.
Place the prawns and chicken in separate bowls. Marinate them with 1 teaspoon soy sauce and 1 teaspoon sake each. Set aside.
Egg Custard
Crack eggs in a large bowl and beat gently with a pair of chopsticks. Whisk in dashi, followed by salt, soy sauce, sake and mirin, taking care not to incorporate too much air bubbles.
Strain the egg mixture into a measuring cup over a fine sieve to remove air bubbles and any eggy lumps. Discard the eggy lumps. Set aside while you prepare the steamer.
Steam
Prepare your steamer - for me, I heat up about 1-inch of water in a pot and set a wire rack inside.
Divide the shiitake mushrooms, prawns and chicken among 4 (or 2 if desired) bowls / teacups / ramekins. Pour in the egg mixture. If there are air bubbles on top, blow at the air bubbles to remove them.
Cover the bowls / teacups / ramekins with aluminum foil and place them in the steamer.
Turn the heat down to low immediately and steam on low heat for 10 to 15 minutes if making 4 portions, 15 to 20 minutes if making 2 portions. Remove the aluminum foil and check for doneness - the top should bounce back when the back of a spoon is pressed very gently on top and clear stock should run after a chopstick is inserted into the centre of the chawanmushi.
Serve
Place the coriander leaves in the centre of the chawanmushi (where you pierced with your chopstick) and serve immediately.
NOTES
This recipe is supposed to be for 4 people as chawanmushi is typically served as an appetiser / starter, and Japanese portions tend to be small. However, we love chawanmushi so much that we can finish them all between X and I.
The ratio of egg to stock is important to get a perfectly silky chawanmushi. The eggs I used weigh about 50 grams each (without shell). If your eggs are small, use one additional egg (and vice-versa).
If you don't have dashi, you can use unsalted vegetable stock or unsalted chicken stock. If you can't get unsalted stock, try to at least use low-sodium stock and reduce the amount of light soy sauce to taste. Do also note that the resulting chawanmushi will taste different.
For the filling, I don't bother par-boiling the chicken / prawns as I cut them into small pieces and they are cooked together with the egg custard. You can also use whatever you want, as long as they are cut into small pieces so that they cook within 15 minutes.
For a luxurious garnish, use uni (sea urchin) or ikura (salmon roe).