Pound the chicken breasts to an even thickness of 1/4" to 1/2".
In a large bowl, whisk together the flour, cornstarch, baking powder, garlic powder, paprika, and black pepper.
Pour the buttermilk into a separate large bowl and add the hot sauce. Stir to combine.
Add the chicken breasts to the buttermilk mixture and marinate for at least 2 hours or overnight.
Preheat the oil in a Dutch oven or high-sided pot to 350°F (175°C).
Remove the chicken from the marinade, allowing any excess to drip off.
Dredge the chicken breasts in the flour mixture, shaking off excess.
Drizzle the wet buttermilk mixture into the dry flour mixture to create "craggy bits" before dredging the chicken again.
Carefully place the coated chicken breasts in the preheated oil. Fry for 4-7 minutes total, or until the internal temperature reaches 160-165°F.
Remove the chicken from the oil and place on a wire rack to drain excess oil.
Assemble the sandwiches by spreading mayo-based sauce on the buns, adding the fried chicken, pickles, and any other desired toppings.