Cook the pasta according to the package directions, but cut the time by one minute. Drain. While the pasta cooks make the sauce.
Heat the olive oil in a medium fry pan. Add the mushrooms and shallots, and cook until the shallots are soft and the mushrooms are beginning to brown.
Add about 1/3 of the stock and simmer for a few minutes until the stock is almost absorbed.
Add the remaining stock and rosemary and cook 4 or 5 minutes.
Add the cream and simmer for a few more minutes. The sauce should thicken slightly. Add the porcini salt and taste. If you think it needs more of the salt add a pinch or two. Add some fresh ground pepper and serve.