Prepare a large bowl of ice water.
Bring a large pot of water to a boil.
Put the basil in a sieve and submerge in the boiling water, stirring, until bright green, 10 to 15 seconds.
Submerge in the prepared ice water; drain on a kitchen towel and squeeze dry.
Salt the boiling water and add the spaghetti; cook until al dente.
Drain, reserving 1 cup of the pasta cooking water; return the pasta to the pot.
Meanwhile, using a food processor, puree the basil leaves, garlic, olive oil, nuts, parmesan, ricotta and ½ teaspoon salt.
Add the pesto to the spaghetti and toss well.
Top with extra basil leaves and serve with more parmesan.