Bring a medium pot of salted water to a boil and cook pasta according to package instructions.
Drain and set aside a small amount of pasta water, just in case.
Add the pasta back the dry pot and add butter.
Combine until melted.
In a medium sized mixing bowl, add heavy cream, mentaiko, and soy sauce.
Gently whisk to combine.
Add the cooked and buttered pasta to the mixing bowl.
Toss to combine thoroughly.
Season with salt and pepper to taste.
If desired, slice scallions, nori, or shiso (ooba) for garnish.
Serve pasta warm and spoon any leftover sauce on top of the pasta.