Marinate the chicken: Place the shio koji, mirin, soy sauce, five spice, ground ginger, sesame oil, white pepper, garlic, scallions, and ginger in the bowl of a food processor and purée until smooth. Transfer the chicken to a baking dish and coat in half of the marinade (remaining marinade will keep, covered and refrigerated, for up to 1 week). Cover with plastic wrap and refrigerate overnight.

Meanwhile, make the dashi mayonnaise. Stir all ingredients together in a small bowl until combined. Cover and refrigerate until ready to use.

Fry the chicken: The next day, heat 2-inches canola oil in a large saucepan until a deep-fry thermometer reaches 360°F.

In a large bowl, whisk the cornstarch and rice flour with 2 cups water. Place the sweet potato starch in a separate bowl. Wipe some of the excess marinade off of the chicken and dip it into the potato starch, then into the dredge. Fry until the chicken is golden and cooked through (a thermometer inserted into the thickest part of the chicken should read 165°F), 7 to 9 minutes. Transfer the chicken to a paper towel-lined plate and season with the cayenne, five spice, and salt.

To serve, butter the insides of the bread with the Japanese mayonnaise. Heat a large cast-iron skillet over medium and toast until golden, about 2 minutes.

In a large bowl, toss the scallions with the cilantro and sesame oil. Place a piece of chicken on the bottom of each bun and top with a ton of the dashi mayonnaise and some of the herb salad. Top with the other bun and enjoy!