Place chicken thighs in a bowl.
In a small bowl, mix buttermilk and hot sauce. Pour over chicken. Turn to coat fully.
Cover and refrigerate for at least 1 hour, preferably overnight.
In a shallow dish, mix flour, cornstarch, garlic powder, onion powder, paprika, salt and black pepper.
Remove chicken from marinade, letting excess drip off.
Dredge thoroughly in flour mixture, pressing to adhere.
Place on a plate and let rest for 10 minutes.
Heat oil in a deep skillet or fryer to 350°F.
Carefully add chicken pieces to hot oil and fry until golden brown and internal temp reaches 165°F, about 4 - 5 minutes per side. Remove to a paper towel-covered plate to drain.
In a small bowl, whisk together all sauce ingredients until smooth. Chill until ready to use. (This can be done in advance. Store in a tightly sealed container, in the refrigerator, until ready to use.)
Butter the brioche buns and toast until golden.
Spread prepared sauce on both sides of the bun.
Add lettuce, pickles and hot fried chicken.
Top with bun and serve hot with fries or slaw.