01 - Bring a large pot of generously salted water to a rolling boil. Add the bucatini and cook until al dente, following the package directions. Reserve approximately 240 ml (1 cup) of the pasta cooking water, then drain the bucatini.
02 - Simultaneously, in a small mixing bowl, cream together the softened unsalted butter and white miso paste until thoroughly combined and smooth. Incorporate the heavy cream, soy sauce, minced garlic, fresh lemon juice, and a generous grinding of black pepper.
03 - Return the drained bucatini to the warm pot set over low heat. Introduce the prepared miso butter mixture to the pasta along with 120 ml (1/2 cup) of the reserved pasta cooking water. Toss energetically using tongs or a pasta fork until the sauce achieves a creamy consistency and coats each strand of pasta. Add more reserved pasta water, a tablespoon at a time, if a looser sauce is desired.
04 - Divide the sauced bucatini evenly among serving bowls. Lavishly garnish with the finely shredded nori, thinly sliced spring onions, toasted sesame seeds, and freshly grated Parmesan cheese, if opting for these additions.
05 - Serve this umami-rich pasta dish immediately while hot.