Bring a medium to large pot of water to a rolling boil with a large amount of salt added to the water (around 1 tablespoon).
Add in the angel hair pasta, stirring immediately to prevent clumping and to encourage even cooking.
Once cooked to al dente, drain the pasta, but reserve about ¾ cup of pasta water.
Add the drained pasta back into the pot, over low or no heat.
Toss in 1 tablespoon of cold butter and stir vigorously (but not so vigorously that it breaks all the noodles) to allow the butter to emulsify as it melts.
Add in the soy sauce and kewpie mayo. Stir.
If the pasta feels dry or difficult to stir, add a small amount of pasta water. The pasta should continuously be saucy and glossy.
Add the next 2 tablespoons of cold butter and stir vigorously again until emulsified.
Once the butter has melted/emulsified, sprinkle in the parmesan cheese and stir once more to combine.
Again, if the pasta is not saucy and glossy, add a bit more pasta water and stir until it is the desired sauciness.
Add salt and pepper to taste.
Serve hot.
Optional, garnish with herbs and more parmesan cheese.