Whisk flour, cornstarch, garlic powder, onion powder, paprika, mustard powder, cayenne, and 2 teaspoons coarse kosher salt in a medium bowl. Whisk egg, buttermilk, bourbon (if using), and hot sauce in another medium bowl. Working with one piece at a time, dredge chicken in flour mixture, making sure to work into crevices. Shake to remove excess; return to rack. Drizzle 3-4 tablespoons buttermilk mixture into seasoned flour and work in with your fingers until small clumps form. Dip chicken into remaining buttermilk mixture, allow excess to drip off, then dredge in flour mixture. Using fingers, press some flour clumps onto chicken so they adhere. Gently shake off excess; return to rack. Chill at least 10 minutes and up to 12 hours.