Bring a large pot of water to a boil. Add the noodles and cook according to package directions until just tender. Reserve ¼ cup of cooking water. Drain the noodles in a colander and set aside.
Microwave spinach with 1 ½ tablespoons water in a large bowl (cover with damp paper towel or microwavable plastic wrap with a small corner left open for steam to escape, or use microwavable steamer). Drain and add to noodles.
Set the pot used for cooking the pasta, over medium heat. Add 2 tablespoons Butter and cook, stirring occasionally until the foam subsides and the butter turns golden brown, about 3-4 minutes. Watch closely so the butter does not burn. Add the black pepper and stir until fragrant. Add 2 tablespoons reserved pasta water, plus noodles and spinach, soy sauce and sugar, and toss until the sauce is thickened and silky. Add more pasta water a tablespoon at a time if needed to ensure the sauce clings to the noodles.
Remove from the heat and season to taste with more soy sauce and black pepper if needed. Divide into two bowls, sprinkle with sesame seeds and serve with chili crisp.