Cook the noodles to al dente, according to the package directions (Reserve ¼ cup of the pasta cooking liquid and drain the noodles)
Add the drained and hot noodles back into the pot they cooked in. Stir in a tablespoon or two of the pasta cooking water
Add the butter and Parmesan and stir until the butter has melted.
Depending on the consistency, stir in another tablespoon or two of the pasta cooking water.
Add salt and freshly ground black pepper to taste.
Stir in the chopped parsley (if using)