In a medium bowl, whisk together the buttermilk, salt, black pepper, and cayenne pepper. Add the chicken breasts, ensuring they are fully submerged. Cover and marinate in the refrigerator for at least 1 hour.

Bread the Chicken

In a shallow dish, combine the flour and breadcrumbs. In another bowl, beat the eggs. Remove the chicken from the marinade, letting excess buttermilk drip off. Dredge each piece in the flour mixture, dip into the eggs, and then coat with the flour mixture again.

Fry the Chicken

In a large skillet, heat the vegetable oil over medium-high heat. Once the oil is hot, carefully add the breaded chicken breasts. Fry for about 6-8 minutes per side, or until golden brown and cooked through. Remove from the oil and drain on paper towels.

Make the Spicy Aioli

In a small bowl, mix together the mayonnaise and sriracha sauce until well combined. Adjust the spice level to your preference.

Assemble the Sandwich

Toast the brioche buns if desired. Spread a generous amount of spicy aioli on the bottom half of each bun. Add fried chicken, top with shredded lettuce and pickles, and place the top half of the bun on. Serve immediately.