Cut each chicken breast in half or butterfly, then flatten with a meat tenderizer until evenly about 1/2 inch (1.25 centimeters) thick. If butterflied, cut the chicken into serving sizes about 5-6 inches (13-15 centimeters) long and 3-4 inches (8-10 centimeters) wide
Place the chicken in a large bowl with the sake, sugar, salt, and pepper, turning to fully coat. Set aside for 30 minutes or refrigerate for 1-2 hours.
Place the flour in an even layer in a shallow, wide bowl or plate.
In another wide bowl, beat together the eggs until completely combined.
Place the panko evenly in a third wide bowl or plate.
In an 8 inch (20 centimeter) skillet or frying pan, pour in vegetable oil until about 1 inch (2.5 centimeters) deep and place over medium low heat.
Remove one of the chicken pieces from the marinade, allowing any excess moisture to drip off.
Coat the marinated chicken piece in flour, shaking and brushing off any excess.
Then, coat the chicken in the egg, allowing the excess to drip off, and finally fully cover the chicken in the panko.
Once the oil is heated to around 340˚F (171˚C), gently place the coated chicken piece in the oil and cook until lightly golden, about 3 minutes.
Carefully flip the chicken and cook the other side until golden and the center reaches 165˚F (74˚C), another 3 minutes.
Transfer the chicken to a wire rack and repeat with the remaining chicken pieces. Allow the cooked chicken to rest for 5 minutes.
Cut the chicken into slices about 1 inch (2.5 centimeters) wide and serve immediately with a Tonkatsu sauce drizzled over the top and alongside cabbage, cherry tomatoes, and rice.