In a small saucepan bring the water to a boil and pour over the prunes. Let the prunes stand for 1 hour, then add the port wine. Cover and let stand for at least 1 hour or overnight, if possible.
Between two sheets of plastic wrap gently pound out the pork cutlets to a thickness of a little more than a 1/2-inch. Season each cutlet with salt and pepper. In a large skillet melt 2 tablespoons of the butter and cook the cutlets over moderately high heat until browned on each side, about 2 1/2 minutes per side. Transfer cutlets to an ovenproof dish, cover with foil and place in a 200 degree F. oven to finish cooking while the sauce is prepared.
To the drippings in the skillet add the shallots and saute for 1 to 2 minutes. Add the liquid from the prunes, bring to a boil and scrape the bits from the bottom of the pan. Simmer the sauce until reduced by half. Add the juices that have collected from the pork cutlets. Add the chicken stock, bring to a boil and reduce until thickened. Reduce heat and stir in mustard and thyme. Season to taste with salt and pepper. Stir in remaining tablespoon of butter and adjust seasoning again, if necessary. Place a cutlet on serving plate, garnish with a few prunes and coat with sauce. Serve immediately.