Gather all the ingredients. I use 2 chawanmushi cups (each can hold up to 200 ml). Please note that the size of the cups matters for cooking time. Also, do not choose thick cups as the heat won't penetrate the cups easily.
To Prepare the Ingredients
In a small bowl, put the dried shiitake mushrooms and the water and soak for 15 minutes. When the shiitake becomes soft and rehydrated, squeeze the water out, cut off the tough stems, and slice the caps thinly. Tip: This soaking liquid is shiitake dashi. If you want to use this shiitake dashi as a part of your dashi in this recipe, remove 2 Tbsp from ½ cup dashi, and replace it with 2 Tbsp shiitake dashi (for 2 servings).
Cut the chicken thigh into ½-inch (1.3 cm) pieces and put them in a bowl. Add the sake and set aside for 15 minutes.
Make a knot in the mitsuba stems (see photo). If you are using a green onion instead, cut it into thin slices.
To Make the Egg Custard Mixture
Whisk the egg in a medium bowl. Add the dashi, mirin, soy sauce, and salt and whisk it all together.
Then, strain the egg mixture through a fine-mesh sieve into another bowl. This helps to get a refined silky texture.
To Assemble the Chawanmushi
Divide all the ingredients into 2 chawanmushi cups: First, add the chicken, shiitake mushrooms, ginkgo nuts, and shimeji mushrooms. Then, put the more colorful ingredients like carrot, kamaboko, and mitsuba on top.
Gently pour the egg mixture into the cups, leaving some ingredients uncovered by the egg mixture. If you see air bubbles, remove them with a spoon or pop with a skewer. Cover with the chawanmushi lid (or aluminum foil).
To Cook the Chawanmushi
In a large pot, bring some water to a boil. There should be enough water to reach halfway up the sides of the chawanmushi cups. Once the water starts boiling, reduce the heat to the lowest setting. Gently place the cups inside the hot water and cover the pot with a lid. Cook for 25-30 minutes on the lowest heat. If you omit the chicken, the cooking time should be only 15-20 minutes. Insert a skewer in the center of the egg custard. If clear liquid comes out, it's done. Serve warm. You can serve it with uni (sea urchin) and ikura (salmon roe) on top. That's how we like to eat chawanmushi.
You can keep the leftovers in a chawanmushi cup and store in the refrigerator for up to 2 days. To reheat, steam for 2 minutes.