Cut the chicken into bite size pieces and place in a bowl.
Grate the ginger and squeeze either with your fingers or with cheesecloth. You want about 1 1/2 tsp of ginger juice. Mix that juice with the soy, sake and mirin and pour over the chicken, stirring to coat. Marinade 10 minutes or so. Drain the chicken, then place a few pieces at a time in the cornstarch and stir to coat. Remove the coated chicken to a plate so they're ready for frying.
Heat the oil until it hits about 350 F. You can test by dropping a small piece of bread in. It should sizzle. Fry the chicken in batches until cooked through, about 2 minutes total. Stir the chicken pieces so all sides get cooked. Drain on a layer of paper towels.
Transfer to a serving dish and garnish with the shiso leaves if you have any. I did not, alas. ;(