Cook Pasta: Fill a large pot with water, add the salt, and bring to a boil. Add the pasta and cook according to the package directions.
Combine with Seasonings: Using a thong, transfer the boiled pasta to a large bowl. Add the olive oil, butter, and soy sauce. Toss until the pasta is evenly coated.
Prepare the Toppings: Open the natto pack and mix it with the attached sauce (if included) until well combined. Crack the egg and separate the yolk from the white. Chop scallions.
Serve: Place the pasta on a plate and top with the natto, egg yolk, and chopped scallions.
Notes
Japanese natto typically comes with a sauce, which we use in this recipe. If yours doesn't include the sauce, you can substitute it with a bit of soy sauce.
If your eggs are not safe to eat raw, please skip it. You can substitute it with soft-boiled eggs. To make soft-boiled eggs, boil water in a small pot, turn off the heat, add the eggs, and leave them in the pot with the lid on for 10 minutes.