Bring a pot of salted water to a boil over high heat. Cook noodles according to the package instructions. Drain and rinse under cold water.
While the noodles are cooking, whisk together gochujang paste, soy sauce, brown sugar, tahini, sesame oil, and 2 Tablespoons water in a small bowl until well combined.
Taste the sauce now! This is where you can adjust the flavoring to make it spicier, add more salt with soy sauce, or even add a touch of acid if you like with a teaspoon or two of lime juice. If you want more umami flavor, add in a little more tahini or you can add in some more sugar if it’s too acidic.
Heat oil in a wok or large skillet over medium-high heat. Once hot, add garlic and cook until fragrant, about 30 seconds. Add sauce and, stirring constantly, cook until thickened, about 2 minutes.
Add noodles, spinach, chicken, and basil. Toss together and cook until spinach is wilted, about 2 minutes.
Serve immediately with your preferred toppings.