Boil pasta in heavily salted water until al dente. Reserve 1/4 cup of pasta water for the sauce. Drain pasta and set aside.
In a large saucepan, melt butter over medium heat. Add cream, soy sauce, and mentaiko. Stir and bring to a gentle simmer. Add pasta water and stir. Add pasta and toss to coat. Garnish with nori and serve immediately.