In a large bowl, whisk buttermilk, hot sauce, 1 tablespoon salt, and several shakes of pepper. Add chicken to buttermilk mixture, making sure chicken is fully submerged. Cover with plastic wrap and refrigerate chicken at least 1 hour or up to 8.
Remove chicken from fridge and mix beaten egg into mixture until combined. Set aside for a moment.
In a large shallow bowl or pie plate, combine flour, cornstarch, 1 tablespoon salt, and 1 teaspoon pepper. Drizzle 2 tablespoons buttermilk marinade over top and toss to combine.
Add the chicken to the flour mixture, turning and tossing in flour mixture until evenly coated. Allow chicken to sit in flour mixture 10 minutes. Meanwhile, spray the grate on a 4-qt air fryer and preheat 3 minutes at 400F.
Working in two batches, spray both sides of two breaded chicken thighs well with pan spray. Place in a single layer in air fryer and fry at 400F 10 minutes until chicken registers 165F, flipping thighs halfway through cooking. Repeat frying process with remaining breaded chicken.