Prepare noodles according to package directions. Before draining, reserve a few tablespoons of the starchy cooking liquid.
While the noodles are boiling, whisk together all ingredients for the sauce in a large, heat-safe bowl.
Depending on the type of peanut butter, the sauce may be thinner or thicker. If it's very thick or lumpy, whisk in a splash of the cooking liquid from the noodles.
When the noodles are cooked, strain them and add them into the bowl with the sauce. Use tongs or chopsticks to thoroughly coat. If the noodles are looking a little dry, add an extra splash of the cooking liquid to thin out the sauce and coat everything nicely.
Taste and adjust sauce to preference. You may wish to add an extra dash of soy sauce for more umami/salt, an extra teaspoon of maple syrup for sweetness, or chili paste for extra spice.
Garnish with additional scallions if desired, along with any optional garnishes you may like, such as chopped peanuts, toasted sesame seeds, fresh cilantro or mint, or a squeeze of fresh lime. Enjoy!