In a mixing bowl, whisk together soy sauce, rice vinegar, brown sugar, cornstarch, water, and sesame oil. Set this flavorful sauce aside.

In another bowl, combine cornstarch and flour, then gently fold in egg whites and cold seltzer water. Let the batter rest.

Heat vegetable oil until it reaches 350°F (175°C). Dip the chicken thighs into the tempura batter and fry in batches for about 5–7 minutes until golden brown. Drain on paper towels.

In a sauté pan, heat a tablespoon of sesame oil and sauté minced garlic and crushed red pepper for about 30 seconds.

Add the fried chicken to the sauté pan with the garlic mixture. Pour in the pre-prepared sauce and toss until fully coated.

Serve immediately for best texture, over rice or with stir-fried vegetables, garnished with sesame seeds.