Melt half the butter in a large sauce pan, add the pancetta and cook for a couple of minutes, to release some of the fat but not allowing it to colour. Add the peas and toss for a couple of minutes. Add 2 ladlefuls of cold chicken stock cover and cook on a low heat until the peas are tender enough to crush. Bring a large pan of salted water to the boil. Add the pasta and cook. While the pasta is cooking, crush the peas. Drain the pasta and add to the sauce. Finish with butter, Parmesan and truffle oil, toss, season and serve.