Whisk buttermilk, mustard, garlic and hot sauce in a shallow glass dish until well blended. Add chicken and turn to coat. Cover and marinate in the refrigerator for at least 1/2 hour or for up to 8 hours.
Preheat oven to 425°F. Line a baking sheet with foil. Set a wire rack on the baking sheet and coat it with cooking spray.
Whisk flour, sesame seeds, paprika, thyme, baking powder, salt and pepper in a small bowl. Place the flour mixture in a paper bag or large sealable plastic bag. Shaking off excess marinade, place one or two pieces of chicken at a time in the bag and shake to coat. Shake off excess flour and place chicken on the prepared rack. (Discard any leftover flour mixture and marinade.) Spray chicken pieces with cooking spray.
Bake the chicken until golden brown and no longer pink in the center, 40 to 50 minutes.