Start by slicing and pounding the chicken until evenly thin. This ensures it cooks evenly and stays tender. Place the chicken in a container and pour the buttermilk over it. Let it marinate in the fridge for several hours or overnight for best results.
In a large bowl, combine the flour, potato starch, and all seasonings. Whisk well so the spices are evenly distributed. Drizzle a few tablespoons of buttermilk into the flour and mix gently to create small clumps, which help form a crunchy crust.
Remove the chicken from the marinade and press it firmly into the breading. Make sure every piece is well coated. Let the breaded chicken rest briefly so the coating sticks better during frying.
Heat oil in a skillet to medium-high heat. Fry the chicken for about 3–4 minutes per side until deep golden and crispy. Transfer to a wire rack to drain while staying crunchy.
Mix melted butter with hot sauce in a bowl. Dip each piece of fried chicken into the buffalo sauce, coating both sides evenly. The sauce should cling nicely without making the crust soggy.
Combine all sauce ingredients in a bowl and stir until smooth. Taste and adjust seasoning if needed. The sauce should be creamy, tangy, and fresh.
Toast the brioche buns with butter until golden. Add lettuce to the bottom bun, place the buffalo chicken on top, and spoon over the blue cheese sauce. Finish with the top bun and serve immediately.