In a large bowl, whisk together the buttermilk, garlic powder, onion powder, smoked paprika, kosher salt, and black pepper. Submerge the chicken thighs in the marinade, cover with plastic wrap, and refrigerate for at least 2 hours or overnight.

In a shallow dish, combine the all-purpose flour, cornstarch, and remaining spices. Whisk together until well mixed.

In a deep skillet or Dutch oven, pour enough vegetable oil to reach a depth of about 2 inches and heat it over medium-high heat until it reaches 350°F (175°C).

Remove the marinated chicken from the buttermilk mixture and dredge each piece in the spiced flour mixture.

Carefully place the breaded chicken thighs into the hot oil without overcrowding the pan. Fry in batches for about 6-8 minutes per side.

Once the chicken is golden and cooked, remove it using tongs and place it on a plate lined with paper towels.

Toast the brioche buns cut side down in a skillet with a small amount of butter over medium heat until golden brown.

Assemble the sandwich by spreading mayonnaise on the bottom half of the bun, layering lettuce, crispy chicken, tomato slices, and pickles, then topping with the other half of the bun.