In a large bowl, combine buttermilk, garlic powder, salt, and black pepper. Add chicken thighs, ensuring they are fully submerged. Cover and refrigerate for at least 1 hour, or overnight for best results.

In another bowl, mix together flour, cornstarch, paprika, and cayenne pepper.

Heat vegetable oil in a deep skillet or fryer to 350°F.

Remove chicken from buttermilk, allowing excess to drip off. Dredge each piece in the flour mixture, pressing lightly to adhere.

Carefully place chicken in the hot oil, frying in batches if necessary. Cook for 5-7 minutes or until golden brown and cooked through (internal temperature should reach 165°F). Remove and drain on paper towels.

In a small bowl, combine mayonnaise, sriracha, and lime juice. Mix well.

Toast brioche buns lightly on a skillet or grill.

Assemble the sandwiches by spreading the sriracha mayo on the bottom bun, adding fried chicken, shredded lettuce, sliced cucumbers, and cilantro. Top with the other half of the bun.

Serve immediately and enjoy!