Cook 1 large bunch cilantro (about 7 oz.), ends trimmed, in a large pot of boiling generously salted water until wilted and dark green, about 45 seconds. Using tongs, immediately transfer to a large bowl of ice water; let sit 1 minute. Drain and squeeze out excess water with your hands. Transfer to a cutting board; coarsely chop. Reserve pot of boiling water.

Transfer cilantro to a blender and add 1 serrano chile, halved, seeds removed, 4 garlic cloves, ½ cup extra-virgin olive oil, 2 Tbsp. fresh lime juice, ¾ cup crumbled Cotija cheese, ¼ cup unsalted roasted pumpkin seeds (pepitas), and 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt. Blend, adding water 1 Tbsp. at a time as needed, until mostly smooth (you want a little texture). Set pesto aside.

Cook 1 lb. spaghetti in reserved pot of boiling water, stirring occasionally, until al dente. Drain pasta, reserving 2 cups pasta cooking liquid. Return pasta to pot. Add 3 Tbsp. unsalted butter and reserved pesto and toss, adding pasta cooking liquid ¼ cup at a time as needed to thin out sauce, until pasta is well coated; season with more salt if needed. Divide pasta among bowls; top with Cotija cheese and crushed unsalted roasted pumpkin seeds (pepitas).

Do Ahead: Pesto can be made 3 days ahead. Transfer to an airtight container; cover and chill.