In a large pot, bring a generous amount of salted water to a rolling boil. Add the spaghetti and cook until al dente, about 8-10 minutes. Reserve 1 cup of pasta water before draining.

While the pasta cooks, heat a skillet over medium heat and add the unsalted butter. Allow it to melt and swirl until golden brown and nutty, about 4-5 minutes.

Lower the heat slightly and add the minced garlic, sauté for about 1 minute until fragrant and lightly golden.

Stir in the white miso paste, mixing until it fully dissolves. Sprinkle in the chili flakes according to your spice preference.

Pour in the reserved pasta water, soy sauce, and rice vinegar into the skillet. Stir continuously for 2-3 minutes as the mixture thickens.

Add the drained spaghetti into the skillet, tossing to coat with the sauce. Add more reserved pasta water if the sauce is too thick.

Incorporate the freshly grated Parmesan cheese, stirring until melted and well combined. Adjust seasoning with salt and black pepper if needed.

Serve immediately, garnished with chopped chives and optional sliced fresh chilies.