Prepare bean curd sheets by carefully breaking into 4-6” segments. Soak in enough water to cover, from two to four hours, until sheets are re-hydrated and pliable.Season pork with salt and pepper to taste. In a large Dutch oven, heat oil over medium high heat. Brown pork evenly, turning as needed. Add chicken stock, soy sauce, mirin, sugar, five spice powder, peppercorns, pepper, ginger, chiles, and black garlic. Return to boil, reduce heat and simmer, covered, for 30 minutes.After 30 minutes, meat should be barely tender. Skim most of the fat from surface. Drain bean curd sheets. Add drained sheets to pot, turning gently to combine. Cover and continue to simmer gently until pork and bean curd sheets are tender, about 45 minutes to one hour. Do not overcook; sheets should still retain their shape and not be falling apart.Carefully drain and remove bean curd sheets and pork from pot to a platter. Strain cooking liquid, discarding solids (including bones). Defat cooking liquid if necessary, and return to pot. Simmer over medium heat until reduced slightly and thickened. Check seasonings.In a large nonstick skillet or pan, heat oil over medium high heat. Carefully add a single layer of pork and bean curd sheets to the pan. Scatter mushrooms around pan. Pan fry until browned on one side, then turn over. Be careful not to tear the bean curd sheets; they will release when browned. Repeat, adding more oil as needed, until pork and bean curd sheets are all pan fried and nicely browned, and mushrooms are cooked through. Remove all to a platter.To serve: Swirl butter into sauce just before serving. Stir until melted. Spoon sauce over pork and bean curd sheets; garnish with chopped green onions. Serve at once with rice and a fresh green vegetable.