Cook your pasta in a pot of well salted boiling water. Reserve a cup of pasta water before draining.
When the pasta has 5 minutes to go, start the sauce. Timing it so the pasta is done right when you need to add it to the sauce will be easier the second time. See notes.
Melt the butter in a large fry pan that will fit all the pasta in it at the end over a medium heat.
When sizzling add ⅓ cup pasta water and the pepper if using and whisk really vigorously. I like to shake the pan with my left hand and whisk with my right.
Let it gently bubble away for 1 - 2 minutes, whisking every so often. It shouldn't be boiling, just a gentle simmer so adjust heat as you need.
Add your cooked pasta in with another ⅓ cup pasta water and toss well. Let it gently bubble for 30 seconds.
Turn the heat down to lowest and add the grated parmesan cheese in an even layer on top of the pasta and leave it alone.
Let it gently melt on top of the pasta for 1 - 2 minutes.
With tongs, swirl the pasta around and around really quickly (watch the video below!) until you get a well emulsified sauce that clings to the pasta. It doesn't take long.
Season to taste with salt and pepper.
Serve straight away and top with more parmesan if you wish and some lemon zest is a great addition.