To make the nanban sauce, add the soy sauce, sake, sugar and ginger to a small saucepan and bring to a boil over high heat. Continue boiling for 1 minute and then turn off the heat and stir in the vinegar.
Fry the chicken until it's golden brown and cooked through (about 6-8 minutes). You'll want to flip the chicken over a few times, but be careful not to break the crust off off the chicken.
Dip the fried chicken in the sauce, flipping it over a few times to coat it evenly.
Slice and serve the chicken with the tartar sauce.