Cook your pasta in a pot of well salted boiling water.
When your pasta is close to done heat a large fry pan over a medium heat and add pasta and ⅔ cup of pasta water.
Whisk well and add your butter piece by piece, letting them dissolve down before adding the next. When all the butter has melted lower your heat slightly.
Add your Parmesan in 2 - 3 lots, again letting each lot melt down before adding the next. Whisk well to combine so all the cheese is off the bottom of the pan.
Let the mixture come up to a very gentle simmer and cook for 2 - 3 mins so it thickens slightly.
Add your cooked pasta directly into the sauce and let it bubble away for another 1 min or so to thicken more (it will thicken up off the heat)
Take off the heat, let it stand for a min or two off the heat so it thickens a bit more then serve!