Put the walnuts in a bowl and cover with boiling water to soften them slightly. Remove with a slotted spoon and use a pestle and mortar to pound them into a purée. Whip in the butter and breadcrumbs. Tip this paste into a bowl, add the cream and season. Bring a large pan of salted water to the boil. Add the pasta and cook according to the packet instructions, until it is al dente. Drain, then put it back in the pan. Scoop the walnut sauce over it and toss until it is coated. Stir in the rocket, finish with the parmesan and parsley, then serve.