Cook the Pasta: Bring a large pot of heavily salted water to a rolling boil. Cook the spaghetti until perfectly al dente. Right before draining, carefully reserve 1/4 cup of the hot, starchy pasta water in a mug.

Prepare the Sauce Base: While the pasta cooks, slit the mentaiko lobes lengthwise. Use a spoon to scrape out the tiny, spicy eggs, discarding the outer membrane. Place the raw eggs in a very large mixing bowl.

Mix the Paste: Add the remarkably soft room-temperature butter, soy sauce, and heavy cream to the bowl with the eggs. Mash everything together with a fork into a smooth, rich paste. (Crucial: Do not put this sauce in a hot frying pan! The delicate roe should only be warmed by the pasta).

Toss and Emulsify: Drain the hot pasta and immediately drop it directly into the mixing bowl with the raw mentaiko sauce. Toss vigorously using tongs, adding a small splash of the reserved hot pasta water. The residual heat will melt the butter and gently cook the roe, creating a beautifully creamy, emulsified sauce.

Assemble: Twirl the glossy pasta onto warm serving plates. Top generously with the fresh julienned shiso leaves and a large pinch of shredded Nori seaweed. Serve immediately.