Whisk together the egg, soy sauce, vodka, buttermilk, baking soda, flour, black pepper, garlic powder, and cayenne in a large bowl. Add the chicken and turn it with your fingers to coat thoroughly. Set aside while preparing the remaining ingredients or marinate in the refrigerator up to overnight.

Combine the chicken stock, oyster sauce, soy sauce, sugar, pepper, sesame oil, and ginger in a small saucepan. Combine the cornstarch and water in a small bowl and stir with a fork until dissolved. Add to the saucepan and stir to combine. Cook over medium heat, stirring, until it boils, reduces, and thickens (5 to 10 minutes). Set the sauce aside and keep warm.

Combine the flour, cornstarch, baking powder, black pepper, garlic powder, cayenne, and salt in a large bowl. Whisk until homogeneous.

Heat the oil in a wok over high heat until it registers 350°F (175°C). Drizzle a few tablespoons of the chicken marinade into the dry coating mixture and work it in with your fingertips to form little clumps of flour. Working one piece at a time, transfer the chicken from the marinade to the dry coating mixture, tossing after each addition to coat. Once all the chicken has been coated, toss with your hands to ensure every piece is thoroughly coated.

Lift the chicken one piece at a time, shake off excess coating, and carefully lower into hot oil. Cook until the chicken is cooked through and crispy, about 4 minutes. Remove with a spider and transfer to a paper-towel-lined bowl to drain.

Optional: Let the chicken cool completely. Strain the oil and discard solids. Reheat the oil to 375°F (190°C), add the rested chicken, and refry until crispy, about 2 minutes.

Add the cashews to the oil and fry until pale golden brown, about 45 seconds. Transfer to the bowl with the chicken.

Transfer the chicken and cashews to a serving platter. Ladle the gravy on top, sprinkle with scallions, and serve immediately with rice.