Cut the chicken thighs into 2 inch (5 centimeter) pieces.
In a large bowl, combine the ginger, garlic, sake, soy sauce, mirin, and sesame oil.
Add the chicken to the bowl and toss well to thoroughly coat in the sauce. Cover and refrigerate for at least 1 hour and up to 4 hours.
In a large wok, bring 2 inches (5 centimeters) of vegetable oil to medium high heat. Once heated and shimmering, reduce heat to medium.
Place the potato starch in a medium bowl. Remove a chicken piece from the marinade and thoroughly coat in the potato starch.
Gently place the coated chicken piece in the hot oil and repeat with remaining pieces. Take care not to overcrowd the pan and cook the chicken in batches if needed.
Fry for 2 1/2-3 minutes per side, until golden and cooked through.
Use a slotted spoon to remove the chicken and drain on paper towel-lined plate. Remove any pieces of batter between batches of chicken.
To assemble the rice bowl:
Fill four bowls with desired amount of steamed rice.
Cover the rice with a layer of shredded cabbage.
Top with the fried chicken pieces, then add a drizzle of mayonnaise.
Serve immediately with desired toppings such as shishito peppers, lemon wedges, green onions, and shichimi togarashi to taste.