In a large bowl, combine the buttermilk, hot sauce, and garlic. Add the chicken thighs and toss to coat. Cover the bowl and refrigerate for at least 3 hours, or up to overnight.
When you're ready to cook, preheat your air fryer to 375 degrees F / 190 degrees C.
In a shallow bowl, whisk together the flour, cornstarch, garlic powder, onion powder, cayenne pepper, and salt.
Remove the chicken from the marinade and dip it in the flour mixture, coating both sides.
Place the chicken in the air fryer basket and cook for 8-10 minutes, or until cooked through and golden brown. Flip the chicken halfway through cooking.
While the chicken is cooking, make the sauce. In a small saucepan, melt the butter over medium heat. Add the cayenne pepper powder, brown sugar, garlic powder, and chili powder. Cook, stirring constantly, for 2-3 minutes, or until the sauce is thickened and smooth.
Remove the chicken from the air fryer and immediately toss it in the hot sauce. Serve hot.