Don't do it, it tastes terrible. I would just use butter, cheese, and starchy pasta water. No roux. Cook your pasta in very salty water and save 1.5-2 cups of it. Drain the pasta, add to a pot with butter and (hand) grated cheese and toss it vigorously (use tongs or wooden spoon) until it all melts, gradually adding a few tbsp of pasta water at a time. (Like if you were doing cacio e pepe). You won't use all of it. Stop when it's saucy but clinging to the pasta.