In a large bowl, whisk together ginger, garlic, sake, soy sauce, brown sugar, and 1 tsp. salt. Add chicken pieces and stir to completely coat. Cover, refrigerate, and let marinate for at least 30 minutes, and up to 1 hour.
Line a baking sheet with paper towels, and set a wire rack inside. Fit a Dutch oven with a candy thermometer, and add oil 2” to 3” deep. Heat over medium-high heat.
Meanwhile, in a large bowl, whisk together flour, corn flour and remaining 1 tsp. salt.
Remove one piece of chicken at a time from the marinade and roll it in the flour mixture to coat. Gently shake off the excess flour mixture and transfer to a large plate. Repeat with remaining chicken.
Place 2 to 3 pieces in the oil and fry until golden brown, about 3 minutes. Transfer chicken to the wire rack. Repeat with remaining chicken.
Garnish with green onions, Japanese mayo, and a squeeze of lemon to serve.