Adjust oven rack 6 inches from broiler element and heat broiler.
Line rimmed baking sheet with aluminum foil.
Brush 2–3 poblano chiles, stemmed, halved, and seeded, all over with 2 teaspoons vegetable oil and arrange skin side up on baking sheet.
Broil until skins are spotty brown and beginning to blacken, 4 to 6 minutes.
Using tongs, flip poblanos and broil until second side is starting to brown and peppers are softened, 4 to 6 minutes.
Let peppers cool slightly, then remove and discard skins.
Process skinned poblanos; 2 cups coarsely chopped cilantro leaves and stems; 8 ounces cream cheese, cut into 8 pieces; 1 cup milk; ¼ cup sour cream; 2 tablespoons chicken bouillon powder; and 1 stemmed and chopped jalapeño chile in blender until smooth, about 1 minute. Set aside.
Bring 4 quarts water to boil in large pot.
Add 1 pound spaghetti and 1 tablespoon table salt and cook, stirring often, until al dente.
Reserve 1 cup cooking water and drain pasta.
Melt 2 tablespoons unsalted butter in now-empty pot over medium heat.
Add sauce and bring to simmer, stirring occasionally, until bubbling and slightly thickened, 3 to 5 minutes.
Add pasta and cook, tossing constantly with tongs, until pasta is evenly coated with sauce.
Adjust consistency with reserved cooking water as needed.
Serve immediately.
