Mix together the ginger, garlic, soy sauce & mirin in a bowl, then add the pieces of chicken & coat thoroughly. Cover with plastic wrap and refrigerate for 10-20 minutes.
Cover a plate or oven tray with absorbent paper towel and place a wire rack over the top. Place the cornflour in a bowl and heat the oil in a high sided saucepan to 180C / 350F.
Remove the chicken from the fridge and one by one, take out pieces of chicken, coat in cornflour and place gently in the hot oil. It's best to do the chicken in 4-5 batches depending on the size of your saucepan. Don't overcrowd the pot as it will cool the oil down too much and won't give you nice crispy chicken bits.
Deep fry each batch for 1 1/2 minutes, then remove and place on the wire rack. Once all batches are complete, starting with the first batch again, place the chicken back into the oil and deep fry a second time for 1 1/2 minutes again. Remove and place on the wire rack until all the chicken is done.
Serve on a bed of rice, with Kewpie mayonnaise on the side and some cucumber and spring onion. Sprinkle the Togarashi spice over the top of the chicken if using.