Bring a large pot of salted water to a boil. Meanwhile, melt 2 tablespoons of the unsalted butter in a large, high-sided skillet over medium heat. Add ½ cup panko breadcrumbs, ¼ teaspoon kosher salt, and ⅛ teaspoon red pepper flakes if using. Cook, stirring regularly, until golden brown, about 3 minutes. Transfer to a small bowl and wipe the skillet clean with paper towels.

Finely grate 1 ounce Parmesan cheese (about ½ packed cup) or measure out ⅓ cup store-bought grated, plus more for serving, if desired. Finely grate 1 garlic clove.

Add 1 pound long dried pasta to the boiling water and cook for 1 minute less than the package instructions for al dente. Reserve 1 ½ cups of the pasta water. Drain the pasta.

Place ¼ cup white or yellow miso paste and 1 cup of the pasta water in a glass measuring cup or small bowl. Whisk until smooth.

Melt the remaining 6 tablespoons unsalted butter in the reserved skillet over medium heat. Add the garlic and cook until fragrant, 20 to 30 seconds. Add the miso mixture and ¼ teaspoon black pepper. Whisk until mostly smooth. Bring to a simmer. Cook, stirring regularly, until slightly thickened, 1 to 2 minutes. Add the Parmesan cheese and whisk to combine. Add the pasta and toss to coat. If the sauce is too thick, thin it out with more pasta water a tablespoon at a time.

Serve topped with the breadcrumbs. Taste and season with more kosher salt and black pepper as needed.