In a large mixing bowl, whisk together buttermilk, fish sauce, and garlic powder. Add chicken thighs and submerge in the marinade. Cover and refrigerate for at least 15 minutes.

In a shallow dish, mix together flour, panko breadcrumbs, salt, and black pepper. Dredge the marinated chicken in the flour mixture, pressing gently to adhere.

Heat vegetable oil in a skillet over medium-high heat until it reaches 350°F (175°C).

Fry chicken thighs in batches for 4-5 minutes per side until golden brown and cooked through. Drain on paper towels.

In a small bowl, mix Thai sweet chili sauce and mayonnaise. Toss shredded cabbage in the sauce until evenly coated.

Toast the brioche buns lightly, then assemble each sandwich with fried chicken, sweet chili slaw, and cucumber slices.