Preheat the oven to 190C/375F/Gas 5.
Warm the milk in a saucepan (with the parmesan rind if using), add the porcini mushrooms, remove from the heat and soak for 10 minutes. Strain through a sieve suspended over a bowl, pressing lightly on the mushrooms with the back of ladle to extract all the milk.
Heat the butter in a clean saucepan, add the flour and stir over a low heat for 2-3 minutes without colouring the roux.
Pour in the porcini-flavoured milk all in one go and whisk together vigorously until smooth. Cook the sauce for a further 10 minutes, or until the sauce has thickened. Season lightly with salt and freshly ground black pepper and set aside.
Bring a large pan of salted water to a rolling boil. Add the pasta and cook until al dente. Drain in a colander, tip into a roomy bowl and carefully stir in the sauce, porcini and pancetta until well combined.
Place the pasta into a lightly buttered oven-proof dish. Smooth the surface and cover with two tablespoons of the parmesan. Bake in the oven for 30-40 minutes, or until bubbling around the edges and golden-brown.
Serve the dish piping hot at the dinner table and have extra cheese at the ready.