Add the ginger, garlic, sake and soy sauce to a bowl and whisk to combine. Add the chicken, then stir to coat evenly. Cover and refrigerate for 2 hours.
Add 2cm of vegetable oil to a heavy bottomed pot or frypan. Heat until the oil reaches 180℃.
Put the potato starch in a bowl. Add a handful of chicken to the potato starch and toss to coat each piece evenly.
Fry the karaage in small batches for 4 minutes, or until the chicken is cooked through. Transfer chicken to the paper towel lined rack to drain. Serve with lemon slices as a garnish.