Cut the chicken and shrimp into bite-size pieces. Sprinkle lightly with soy sauce
Prepare four cups and add some of the shrimp, chicken and shitaki to each of them. Slowly pour the egg mixture into each cup.
Boil water in a steamer. Cover each cup tightly with aluminum foil. Place the cups in the steamer. Cook for 15 minutes, until the custard is just set; it will still be very soft. Do not overcook.