Spaghetti with parsley pesto

From Bon Appetit:

1 pound spaghetti
Kosher salt
1/2 cup unsalted, roasted almonds
4 cups (packed) fresh flat-leaf parsley leaves
3/4 cup chopped fresh chives
3/4 cup extra-virgin olive oil
1/2 cup finely grated Parmesan
Freshly ground black pepper

Cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente. Drain, reserving 2 cups pasta cooking liquid. Meanwhile, pulse almonds in a food processor until smooth. Add parsley, chives, oil, and Parmesan; process until smooth. Season pesto with salt and pepper. Toss pasta and pesto in a large bowl, adding pasta cooking liquid by 1/4-cupfuls until saucy. Season with salt and pepper.
DO AHEAD: Pesto can be made 5 days ahead. Cover surface directly; chill.