Prepare the Chicken Batter: Pat the chicken pieces very dry with paper towels. In a large bowl, whisk together the cornstarch, flour, baking powder, salt, and pepper. Pour in the cold water and whisk until a smooth, thin batter forms. Add the dried chicken pieces to the batter and toss until every piece is thoroughly coated. Set aside.

Fry the Chicken: Heat about 1.5 inches of oil in a heavy-bottomed pot or deep skillet over medium-high heat to 350-375°F (175-190°C). Working in batches to avoid crowding, carefully add the battered chicken pieces. Fry for 3-4 minutes per side until deeply golden brown and cooked through. Transfer the cooked chicken to a wire rack set over a baking sheet to drain and stay crispy. Repeat with remaining chicken.

Make the Sauce: Carefully pour out all but 1 tablespoon of oil from the pan. Return the pan to medium heat. Add the minced garlic and grated ginger, stirring for 30 seconds until fragrant. Add the tomato paste and cook for another 30 seconds. Pour in the rice vinegar, soy sauce, honey, brown sugar, and sriracha. Whisk together and bring to a simmer.

Thicken the Sauce: Give the cornstarch-and-water slurry a final stir and drizzle it into the simmering sauce while whisking constantly. Continue to whisk until the sauce thickens and becomes glossy, about 60 seconds. Remove from heat and stir in the toasted sesame oil.

Combine and Serve: Add all the crispy chicken pieces to the pan with the sauce. Using a gentle folding motion, quickly coat every piece in the sauce. Transfer to a serving platter, garnish immediately with sliced green onions and sesame seeds, and serve hot with cooked rice.