Heat the peanut butter for 10 seconds in a microwave to make it easier to mix. Combine with the soy sauce, vinegar, hot red pepper oil, cayenne, half the garlic and ginger, salt and pepper. Whisk together. Whisk in the sesame oil and broth. Set aside.
Heat the canola or peanut oil in a wok or a large, heavy skillet over medium-high heat, and add the snow peas. Stir-fry for one to two minutes, and add the scallions and remaining garlic and ginger. Stir-fry for 20 seconds, and add the tofu (if using). Stir-fry for one to two minutes, then stir in the noodles and sauce. Toss together until the noodles are hot, and remove from the heat. Add the radishes and cilantro, stir together, and serve.
Variation: Substitute thinly sliced cooked chicken breast for the tofu.
Advance preparation: You can cook the noodles up to three days ahead. Toss them with 1 teaspoon canola oil and refrigerate. The ingredients for the sauce can be combined several hours before you make the stir-fry.