Truth be told, I am not a super fan of Kewpiemayo. It has its place and time, but generally in my recipes when I call for mayo I am thinking of something not super seasoned or strong in flavor, more of a light sweetness that will let other ingredients shine. Like when making a ponzu or miso mayo withKewpie, it is too salty in my opinion.
Process the egg, egg yolk, Dijon mustard, rice vinegar, liquid shio koji and salt together in a food processor (or blender) for about 30 seconds. With the machine running, slowly add the oil, being careful not to break the mayonnaise. If it gets too thick, add water 1 tablespoon at a time to loosen it up.