Turn broiler to high. Place peppers on baking sheet and broil, turning occasionally with tongs, until blackened on all sides. Transfer chilis to a heat-proof bowl and cover with plastic wrap. Let peppers steam for 10 minutes, then remove plastic wrap and peel skins off peppers. Remove stems and seeds and roughly chop peppers.

Combine peppers, cilantro, onion, and garlic into a food processor or blender and blend until smooth. Then add butter and cream cheese.

Meanwhile, make spaghetti: In a large pot of boiling salted water, cook spaghetti according to package instructions. Reserve 1 cup pasta water then drain.

Mix sauce into cooked pasta, drizzling with reserved pasta water as needed. Adjust to taste with salt and pepper.