Bring a large pot of salted water to the boil and cook spaghetti according to packet instructions. When pasta is almost cooked carefully get a cup full of boiling cooking water into a jug or mug and set aside

When pasta is nearly ready, melt butter in a large pan over low heat. Once cooked, add drained pasta, reserved pasta water, grated cheeses and a generous grinding of black pepper (about 1 tsp) and season to taste with salt. Mix together gently until pasta is coated. Serve immediately