Bring a large pot of water to boil. Once boiling, add 2 Tbsp kosher salt and cook the spaghetti until al dente, about 10 minutes (check the package instructions as well).
In a large bowl, combine 2 Tbsp milk, 2 Tbsp heavy whipping cream, 2 Tbsp unsalted butter, and 1 Tbsp soy sauce. Don't worry if the butter becomes solidify. Hot spaghetti will melt the butter again.
Add freshly ground black pepper and stir to combine.
Cut each roe sac in half and squeeze out the roe from the sac with hands or knife.
Add the roe to the bowl and mix well. Discard the membrane.
When the spaghetti is done, drain and transfer to the large bowl with the sauce.
Toss to combine until the butter is melted and the sauce is evenly distributed. Taste and adjust with black pepper and kosher salt.
Serve immediately, garnished with shredded nori. If you can find shiso leaves in a Japanese grocery store, roll them up and cut it into thin strips and garnish. Shiso is a wonderful addition to this dish.
Unfortunately, you can't keep the leftovers. If you reheat mentaiko, the fish eggs will be cooked/overcooked.