Combine chicken with with ginger, soy sauce, sake and salt and toss until thoroughly combined. Let sit in the fridge for at least 30 min or up to 24 hours.
Combine starch and flour in a large bowl. Add sesame seeds and mix until combined. When chicken is ready, remove from the marinade and coat in the starch mixture, knocking off any excess. Repeat with all pieces of chicken.
Add oil to a large pan about an inch up the side. Let oil get to 350 degrees and then add in the pieces of chicken. Cook until crispy, about 4 min per side.
Make the kewpie mayo and slaw by combining ingredients and season to taste.
Toast bun and add lemon mayo, and a piece of chicken. Give the chicken another nice squeeze of lemon. Add slaw and enjoy!