Bring a large pot of salted water to the boil.
Heat a small, dry frypan on medium heat. Add pine nuts and cook for 1-2 minutes, shaking pan often, until golden and toasted. Remove from pan and set aside to serve.
With your hands, separate pasta so it is not stuck together. Cook pasta in pot of boiling water for about 3 minutes, until just tender. Reserve 1/2 cup pasta water, then drain well.
Return pasta to pot with pesto and 1/4 cup of reserved pasta water. Toss together to coat pasta. Add a splash more pasta water, if needed (the pasta water helps to create silky pasta and prevents it from being dry and gluggy). Season to taste with salt.
Top pasta with pine nuts.