In a large bowl, whisk together buttermilk, 1 teaspoon salt, and 0.5 teaspoon black pepper. Submerge chicken thighs and refrigerate for 30 minutes.

In a separate shallow dish, combine flour, cornstarch, and baking powder.

In a small saucepan over medium heat, whisk together gochujang, soy sauce, honey, rice vinegar, ginger, garlic, and sesame oil. Simmer for 3 to 5 minutes until slightly thickened, then set aside.

Heat vegetable oil in a heavy-bottomed pot to 350°F (175°C).

Remove chicken from buttermilk, allowing excess to drip off, and dredge thoroughly in the flour mixture, pressing to create crags.

Fry chicken in batches for 6 to 8 minutes or until the internal temperature reaches 165°F (74°C) and the crust is golden brown.

Drain chicken on a wire rack for 1 minute, then toss in the warm gochujang glaze until fully coated.

Toast brioche buns and assemble sandwiches by placing glazed chicken on the bottom bun and topping with kimchi slaw.