Combine chicken thighs, ginger, garlic and soy sauce in a mixing bowl. Cover and chill in the refrigerator for at least 1 hour, up to 4 hours.
Remove the marinated chicken from the refrigerator 30 minutes before cooking to let them come back to room temperature.
Prep Work
Place corn starch / potato starch on a shallow bowl. Line and bowl with paper towels to absorb excess oil. Set aside.
Heating the Oil
Heat 2-inch deep of oil in a pot / wok over medium heat until it reaches 160-170°C / 320-340°F.
If you don't have a thermometer, insert a wooden spoon or a pair of chopsticks in the oil - if the oil bubbles vigorously around the spoon / chopsticks, the oil is ready for frying.
The First Fry
Once the oil is ready, coat a few chicken pieces (I do 5 or 6 at a time) with the corn starch / potato starch and add them into them hot oil.
Fry the chicken pieces for about 2 to 3 minutes, flipping frequently, until lightly browned on all sides. If the chicken pieces are browned too quickly, reduce the heat.
Remove the cooked chicken pieces with a slotted spoon, drain well and transfer to the bowl lined with paper towels.
Use a metal mesh strainer to scoop up any bits of flour or smallish chicken bits so they don't burn in the oil.
Repeat until all the chicken pieces are fried.
The Second Fry
Turn the heat back up to high and heat the oil until it reaches 180°C / 360°F. The oil should bubble even more vigorously around the wooden spoon / chopsticks but not smoking. If the oil is smoking, the oil is too hot. Remove from heat and let cool slightly before using.
Add all the chicken pieces back into the hot oil and cook for another 30 seconds to 1 minute for the chicken pieces to expel the excess oil absorbed by the chicken pieces.
Drain the chicken pieces well and transfer them to the bowl lined with paper towels.
Transfer the chicken pieces to a serving plate and serve with the Dipping Sauce.
Dipping Sauce
Stir mayonnaise, sriracha and lemon juice together in a small bowl. Adjust the sauce according to your liking.
NOTES
Note that cornstarch = corn flour but potato starch is also not the same as potato flour!
Frying the chicken in batches will prevent the oil temperature from dropping too much, resulting in oily chicken.