Brine. Whisk buttermilk, hot sauce, egg, salt, pepper, garlic powder. Submerge chicken. Refrigerate at least 4 hours, ideally overnight.
Air-dry. Drain chicken, place on wire rack over a baking sheet, refrigerate uncovered 30 minutes.
Mix breading. Whisk flour, cornstarch, baking powder, and seasonings.
Heat oil to 350°F (1 inch deep in a skillet or Dutch oven).
Bread chicken. Press each fillet into breading, turning to coat. Press firmly. Let rest 1 minute on rack.
Fry 2 fillets at a time, 3–4 min per side, until 165°F internal and deep golden brown.
Drain on a wire rack — never paper towels.
Toast buns butter-side down in a dry skillet, 1 minute.
Build: bottom bun → 1 tbsp mayo → fried chicken → 4 pickle slices → top bun.
Serve immediately.