Bring a large pot of salted water to a boil. Add pasta and cook according to package directions until al dente. Before draining, reserve about 1.5 cups of the starchy cooking water.

While the pasta cooks, melt butter in a large skillet over medium heat. Add the minced garlic and cook until fragrant, about 30-60 seconds.

Reduce heat to low. Add the white miso paste to the skillet and whisk it into the butter. Slowly pour in about 1/2 cup of the hot reserved pasta water, whisking constantly until the miso is fully dissolved and the sauce is smooth.

Drain the pasta and immediately add it to the skillet along with the grated Parmesan cheese. Toss vigorously with tongs, adding more pasta water a little at a time, until the sauce becomes glossy, creamy, and coats the pasta perfectly.

Serve immediately, garnished with sliced scallions, toasted sesame seeds, or a pinch of red chili flakes, if desired.