Cut off the stems of the shiso leaves and slice them thinly.

Make a cut in the mentaiko and extract the contents.

In a bowl, combine mentaiko, butter, soy sauce, and pepper.

Boil water in a pot, add salt (not listed in ingredients), and cook spaghetti 1 minute less than the package instructions. Reserve 2 tbsp of pasta water and drain the rest.

Add the spaghetti and reserved pasta water to the bowl with the sauce. Mix well until evenly coated.