Snap off the stem end of the green beans, then snap beans in half to make the beans bite-sized, if desired. Blanch green beans in generously salted, boiling water until bright green, about three to five minutes. Scoop out green beans with a sieve, then quickly cool in an ice bath. Return water to boiling, add pasta, and cook following instructions on box. Reserve half of a cup of pasta water, and strain pasta. Return pasta to pot off the heat. Toss pasta with green beans and pesto. Add a splash of pasta water to loosen the pesto and coat the pasta. If desired, top with drizzle of olive oil and sprinkling of parmesan.