Prepare the chicken: Pat the chicken pieces completely dry with paper towels. In a large bowl, whisk together the potato starch, flour, salt, pepper, and baking powder.
Make the batter: Slowly whisk in the cold water until you have a thin, smooth batter, similar to crepe batter. It should coat the back of a spoon thinly.
Coat the chicken: Dip each piece of chicken into the batter, allowing excess to drip off. You want a thin coating, not a thick layer.
First Fry: Heat about 3 inches of vegetable oil in a deep, heavy-bottomed pot or Dutch oven to 325°F (160°C). Fry the chicken in batches for 8 to 10 minutes until lightly golden and cooked through. Remove and drain on a wire rack. Let the chicken rest for at least 15 minutes.
Make the glaze: While the chicken rests, combine all glaze ingredients (gochujang, soy sauce, honey, vinegar, sugar, garlic, ginger, sesame oil) in a small saucepan. Bring to a simmer over medium heat, stirring constantly until the sugar dissolves and the sauce thickens slightly, about 3 minutes. Remove from heat.
Second Fry: Increase the oil temperature to 375°F (190°C). Fry the rested chicken pieces again for 2 to 3 minutes until they are deep golden brown and extremely crispy. This step creates the signature crunch. Drain well.
Toss and Serve: Place the hot, double-fried chicken in a large, clean bowl. Pour the warm glaze over the chicken and toss quickly until every piece is evenly coated.
Serve immediately, garnished with sesame seeds and scallions.
Notes
For the crispiest result, ensure your oil temperature is accurate for both frying stages.
If you prefer a soy garlic flavor, substitute the gochujang glaze ingredients with 1/2 cup soy sauce, 1/4 cup water, 1/4 cup sugar, 2 tablespoons minced garlic, and 1 tablespoon ginger, simmered until reduced.
Do not overcrowd the pot during frying; work in small batches to maintain oil temperature.