Slice any excess fat from the chicken thighs. Cut chicken into bite-sized pieces, about 1 or 2 inches.
In a large mixing bowl or ziploc bag, combine ginger, garlic, soy sauce, sake, sesame oil, and brown sugar. Stir until the sugar has mostly dissolved.
Add chicken thighs to the marinade, cover/seal, and store in the fridge for 2 hours or overnight.
When your chicken is finished marinating, fill a deep pan or pot with enough oil to deep fry (I used an entire 32oz bottle of vegetable oil). Heat to 350F.
In another large mixing bowl, combine eggs, flour, and ice cold water.*
Coat a piece of chicken in the batter and gently drop it into the hot oil. Fry for 3-5 minutes until crispy and golden brown. Remove from the oil and place on a tray lined with paper towels.
Repeat until all chicken has been fried once.
By the time you have fried all of your chicken, the first piece you deep fried should be cooled down significantly. Repeat step 6 going in order from the first chicken you fried to the most recent to ensure that you are not double-frying extremely hot chicken.
Pat chicken dry with paper towels and serve with your favorite dipping sauce!