Marinate Chicken: In a large mixing bowl, whisk together buttermilk, soy sauce, sriracha, garlic powder, ginger powder, and smoked paprika. Add chicken thighs, cover, and refrigerate for 2 hours or overnight.

Prepare Sriracha Mayo: In a small bowl, combine mayonnaise, sriracha, lime juice, and garlic powder. Cover and refrigerate until ready to use.

Make Slaw: Toss together shredded cabbage, chopped pineapple, diced red onion, and sliced scallions. Whisk dressing ingredients in a small bowl, combine with slaw, and refrigerate for 30 minutes.

Cook Glaze: In a saucepan, combine teriyaki sauce, pineapple juice, brown sugar, grated ginger, and minced garlic. Simmer for 5 minutes, mix cornstarch slurry and add, cooking until thickened.

Coat Chicken: In a shallow dish, whisk together flour, cornstarch, baking powder, salt, pepper, and onion powder. Dredge marinated chicken, allowing it to rest on a wire rack for 10 minutes.

Fry Chicken: Heat oil to 350°F. Fry chicken thighs for 6-8 minutes until golden brown. Transfer to wire rack to drain excess oil.

Assemble Sandwich: Brush fried chicken with glaze. Toast brioche buns, spread sriracha mayo, layer with chicken, grilled pineapple, and tropical slaw. Serve immediately.