In a shallow bowl, combine flour, salt, pepper, and chili powder.
In a second bowl, whisk together buttermilk, eggs, and sriracha.
Dredge each piece of chicken in the flour mixture, pressing to coat. Shake off excess.
Dip into the buttermilk mixture, then dredge again in the flour mixture.
Heat about 2 inches of oil in a large pot over medium-low heat until it reaches 350°F.
Fry chicken in batches, 4-5 minutes per side, or until golden brown and the internal temperature reaches 165°F.
Transfer to a paper towel-lined plate.
Lightly toast the brioche buns.
In a small bowl, mix mayonnaise and sriracha. Assemble sandwiches by layering tomato, fried chicken, lettuce, and pickles on the bottom bun. Drizzle with sriracha mayo and top with the remaining bun before serving.