Bring a large pot of well salted water to a boil. Meanwhile, wash the bok choi and separate the leaves. Drop the bok choi leaves into boiling water and blanch for 1 to 2 minutes, until barely cooked. Drain and spread out on a towel-lined baking sheet to cool, then arrange the leaves in one layer in a shallow baking dish and set aside. Put a large wok or cast-iron skillet over high heat. Add the vegetable oil and heat until nearly smoking, then add the chiles and shiitake caps, stirring to coat. Season lightly with salt and black pepper and stir fry for 2 minutes. Reduce the heat slightly, add the garlic, ginger, sugar, sesame oil, and tamari, and stir fry for 1 minute. Spoon the shiitakes and pan juices over the bok choi. Garnish with scallions and sesame seeds.