Separate egg yolks from whites, and set aside 1
In a saucepan, add just enough water to cook pasta and bring to boil. Then cook noodles until a bit before preferred doneness. Reserve some pasta water, and drain the rest out 2
In same pan over low/med-low heat, combine soy sauce and mirin. While stirring, add starch slurry.3 Then, add yolks and thoroughly mix to incorporate
1/2 tbsp potato starch (mixed with + 1 tbsp water to make slurry)
Continue stirring slowly but constantly for a few minutes, essentially lightly and evenly cooking yolks (sign it's cooked the right amount: sauce goes from pooling at bottom of pan to leaving streak marks, but easily wiped away with a stir of the noods)
1.5 tbsp soy sauce, 1.5 tbsp mirin
Thoroughly mix in a few tbsp of pasta water to loosen / add volume to the sauce
Off the heat, mix in white pepper and a drizzle of sesame oil to taste