Slice each chicken breast in half horizontally to create 4 cutlets total. Place each cutlet in a resealable bag and pound to an even ¼-inch (6mm) thickness with a meat mallet.
Whisk together buttermilk, hot sauce, garlic powder, onion powder, smoked paprika, salt, and pepper in a bowl. Add the pounded chicken and refrigerate for at least 2 hours or up to 24 hours.
Combine flour, cornstarch, baking powder, garlic powder, onion powder, smoked paprika, cayenne, salt, and pepper in a shallow bowl. Drizzle in 2 tablespoons buttermilk and work it in with your fingers to create small, shaggy clumps.
Remove chicken from marinade, letting excess drip off. Press firmly into the flour mixture on both sides, then place on a wire rack. Rest for 15-20 minutes at room temperature.
Heat 2-3 inches (5-7.5cm) of vegetable oil in a Dutch oven to 350°F (175°C). Prepare a wire rack over a paper towel-lined baking sheet for draining.
Fry cutlets one at a time for 3-4 minutes per side until deep golden brown and internal temperature reaches 165°F (74°C). Transfer to a wire rack to drain and rest for 2-3 minutes.
Combine buffalo sauce and melted butter in a small saucepan over low heat. Whisk until warm and cohesive, about 2 minutes.
Dip each fried cutlet into the warm buffalo sauce mixture, turning to coat both sides evenly. Set back on the wire rack to let excess sauce drip off.
Toast brioche buns cut-side down in a skillet over medium heat for 1-2 minutes until lightly golden. Spread blue cheese or ranch dressing on the bottom bun, add lettuce, chicken, tomato, pickles, and blue cheese crumbles.