Bring a large pot of salted water to a boil. Add the noodles and cook as the label directs. Reserve 1/2 cup cooking water, then drain. Whisk the peanut butter, 2 tablespoons each water and soy sauce, 1 tablespoons sesame oil, the carrots and half of the jalapeno in a large bowl. Add the noodles and toss to coat.
Meanwhile, heat the remaining 2 tablespoons sesame oil in a large skillet over medium-high heat. Add the scallion whites, ginger and garlic; cook, stirring until golden, about 2 minutes. Add the pork; cook, breaking up the meat with a wooden spoon, until browned, about 5 minutes. Stir in the remaining 2 tablespoons soy sauce.
Transfer the pork mixture to the bowl with the noodles. Add half of the scallion greens and cilantro and toss to coat, adding enough of the reserved pasta water to loosen. Serve topped with the remaining jalapeno, scallion greens, and cilantro.