Cook pasta in a large pot of salted boiling water until al dente, according to package directions, reserving ⅓ cup of the pasta water. Drain.
Set a small nonstick skillet over medium-low heat. Add the sesame oil and most of the scallion whites (reserving some for garnish) and sauté until tender. Add ginger and garlic and cook until fragrant, 20 seconds. Mix in the peanut butter, soy sauce, water, vinegar, sriracha, brown sugar, and scallion greens. Remove from heat.
Toss noodles with the sliced carrot and sauce, adding in some of the reserved pasta water only if necessary to loosen it up a bit (it will set up pretty fast.)
Serve immediately garnished with crushed peanuts and some scallions.
