In a large bowl, whisk together broth, peanut butter, soy sauce, honey, garlic, ginger, and sriracha and red pepper flakes, if using.
Cook soba noodles in boiling water for about 4 minutes. Drain.
Slice baby bok choy in half lengthwise. Heat a frying pan over medium-high heat and lightly coat with nonstick cooking spray. Place bok choy cut-side down and cook for 2-3 minutes with a lid on.
Add noodles, bok choy, carrots, and scallions to the bowl with the sauce and toss to coat.