Bring a pot of water to boil. Add in the pasta and cook until al dente. Pour 1/2 cup of water in a mug and drain the pasta.
Melt butter in the same pot and put the drained pasta back into the pot. Add in grated parmesan and salt. Give it a stir and pour the pasta water over it. Stir again and cook for about a minute or until everything looks combined and there is almost no water.
Serve topped with chopped green onion and extra parmesan if you like.