Start the Sauce: In a medium-sized, heat-proof bowl, combine the minced garlic, chopped scallion, chili flakes, white sesame seeds, and Chinese sesame paste. Set it right next to your stove.
Make the Aromatic Oil: Place a small wok or skillet over high heat. Add the 3 tablespoons of neutral oil and heat it until it's shimmering and just beginning to smoke. Carefully and immediately pour the hot oil directly over the ingredients in the heat-proof bowl. It will sizzle and bubble—this is exactly what you want! Quickly stir everything together with a whisk or chopsticks. The heat from the oil will toast the spices and release their incredible aroma.
Finish the Sauce: To the same bowl, add the light soy sauce, Chinkiang vinegar, sugar, and salt. Whisk everything together until the sauce is smooth and well combined. Set aside.
Cook the Noodles: Bring a medium pot of water to a rolling boil. Add the noodles and cook according to the package directions, usually about 3-4 minutes for fresh noodles. Cook until they are just tender with no raw white center, but still have a pleasant chew. Be careful not to overcook them, as they will become mushy.
Cool and Drain the Noodles: Once cooked, immediately drain the noodles in a colander and rinse them thoroughly under cold running water. This stops the cooking process and washes away excess starch. Shake the colander well to remove as much water as possible.
Prevent Sticking: Transfer the cool, drained noodles to a mixing bowl. Pour the sesame oil and Sichuan peppercorn oil (if using) over the top and toss well to coat every strand. This crucial step prevents the noodles from clumping together.
Assemble and Serve: Add the prepared sauce to the bowl with the noodles. Top with the julienned cucumber and chopped cilantro. Toss everything together until the noodles are thoroughly coated in the delicious sauce. Garnish with crushed peanuts if desired, and enjoy immediately!