Bring a large pan of salted water to a boil, add sprouts, and cook them 2 minutes to blanch them. Drain and rinse them with cold water. Melt butter in a wok. Stir in the sprouts with a little salt and pepper, cover and turn the heat very low. Add salt and pepper and cook the sprouts, stirring them occasionally, at least 30 minutes and preferably longer.
At the end of cooking, remove the lid, raise the heat and cook until the sprouts are dry and just start to brown. They should be meltingly tender and rich.