Cook the noodles. Bring a large pot of water to a boil. Cook the noodles according to the package instructions until al dente.
Prep the sauce base. While the noodles cook, mince the garlic. In a small bowl, stir together the soy sauce, water, and sugar until the sugar dissolves. Set aside.
Cook the garlic. In a 10- to 12-inch sauté pan, melt the butter over medium heat. Add the garlic and a small pinch of salt. Stir until fragrant and softened, about 15 seconds. Do not let the garlic brown.
Build the chili garlic sauce. Add the chili garlic sauce, miso paste or nut butter, and the soy sauce mixture. Stir with a wooden spoon until the sauce is smooth, slightly thickened, and well combined, about 1 minute. Turn off the heat while the noodles finish cooking.
Toss the noodles. Reserve ¼ cup of the noodle cooking water, then transfer the noodles directly into the pan with the sauce. Turn the heat to medium-high. Add the reserved noodle water and toss until the noodles are well coated and the sauce looks glossy.
Finish and serve. Transfer the noodles to a large serving plate. Top with scallions, sesame seeds, and cilantro, if using. Drizzle with a little more chili garlic sauce, toss, and serve right away.
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Notes
Noodle water: If using Asian wheat noodles, you usually do not need to salt the boiling water because many Asian wheat noodles are already salted. Rice noodles are usually not salted, so you can lightly salt the water before boiling.
Chili garlic sauce amount: The chili garlic sauce measurement is based on my homemade version, which is flavorful, smoky, lightly sweet, and less spicy than many store-bought chili oils or sauces. If using store-bought chili oil or sauce, the heat level can vary a lot. Start with 1 tablespoon, then add more to taste.
Make-ahead: The homemade chili garlic sauce can be made ahead and stored in the fridge for up to 1 month.
Storage: Store leftover noodles in an airtight container in the fridge for 2–3 days.
Reheat: Add a splash of water before reheating to loosen the sauce and help the noodles turn glossy again.