Season chicken with salt, pepper, paprika, and garlic powder.
Soak chicken in buttermilk (with hot sauce if using) for at least 20 minutes or up to 4 hours.
In a bowl, mix flour, cornstarch, baking powder, and a pinch of salt.
Heat oil in a deep skillet to 350°F (175°C).
Remove chicken from buttermilk and coat thoroughly in flour mixture, pressing lightly.
Fry chicken 4–5 minutes per side until golden and cooked through (165°F internal temp).
Drain on paper towels and let rest 3–5 minutes.
Spread mayo on toasted buns, add lettuce, tomato, pickles, and crispy chicken.
Close sandwich and serve immediately.