Whisk the 1 cup buttermilk, 1 tbsp hot sauce, and 1 tsp garlic powder together in a bowl. Add the chicken thighs, cover, and refrigerate for at least 30 minutes.

Stir the 1.5 cups flour, 2 tbsp cornstarch, 1 tsp cayenne, 1 tsp smoked paprika, 1 tsp onion powder, 1 tsp salt, 0.5 tsp black pepper, and 0.5 tsp white pepper together in a shallow dish until evenly mixed.

Lift each thigh from the buttermilk, press firmly into the flour, dip briefly back into the buttermilk, press into the flour again, and rest on a wire rack for 10 minutes.

Heat neutral oil to 350°F (175°C) in a deep pan. Fry the chicken for 6 to 7 minutes on the first side, flip, and fry for another 5 to 6 minutes until the internal temperature reads 165°F (74°C). Transfer to a wire rack.

Stir the 2 tbsp mayonnaise and 1 tsp sriracha together. Toast the 2 brioche buns cut-side down in a dry pan over medium heat for 2 minutes. Spread the sriracha mayo on both cut sides, layer 2 lettuce leaves and 2 pickle slices on the bottom bun, place the hot fried chicken on top, and press the lid on. Serve immediately.