Bring 3-4 quarts of water to a boil in a large pot. Stir in the salt. Add spaghetti. Cook until tender, about 2 minutes for fresh or 10 minutes for dried. Remove pasta with tongs into a bowl. Set aside. Reserve pasta water.
In a large shallow frying pan over low heat, melt 1 1/2 tablespoons of butter and stir together with black pepper. It will take about a minute. Add pasta water, turn the heat down, and simmer. Add cooked pasta and remaining butter. Add Parmesan cheese, stirring and tossing with tongs until the cheese is melted.
Turn the heat off and add Romano cheese and lemon zest if using, turning with tongs. Stir in more pasta water if the sauce is too dry. Serve immediately.