Fill a large pot or Dutch oven with water and stir in kosher salt. Bring to a boil, add the spaghetti, and cook until al dente, about 10 minutes or according to package directions. Reserve 1 cup of pasta water before draining the spaghetti.
While the pasta is cooking, preheat the broiler. Line a large baking sheet with aluminum foil and arrange the poblano peppers in a single layer on top. Broil for 8 to 10 minutes, flipping halfway through, until the skin is blackened and blistered.
Loosely cover the baking sheet with aluminum foil or plastic wrap, and let them steam for 5 minutes. Rub off and discard as much of the charred skin as possible.
Transfer the poblanos to a cutting board, slice open, and discard the stems and seeds.
Add the roasted poblanos, cream cheese, Mexican crema, onion, garlic, and chicken bouillon to a large blender. Blend until completely smooth.
Melt the butter in the same large pot or Dutch oven over medium heat. Pour in the poblano cream sauce and cook for 3 minutes, stirring occasionally.
Add in the pasta and toss to coat. Cook for 1 more minute, then taste and season with more salt if needed. If the sauce dries out a little or becomes too thick, mix in a splash of the reserved pasta water.
Remove from heat and serve in bowls topped with queso fresco and fresh cilantro.
Notes
Peppers: This recipe is not spicy. If you’d like to add some heat, roast a jalapeño or serrano pepper in addition to the poblanos.
Mexican crema: If you can’t find Mexican crema, use heavy cream instead.