In a large bowl, combine buttermilk, garlic powder, paprika, salt, and black pepper. Add the chicken breasts, ensuring they are fully submerged. Cover and refrigerate for at least 1 hour, or overnight for best results.

In a shallow dish, mix together flour, breadcrumbs, and cayenne pepper (if using). Remove the chicken from the buttermilk marinade, allowing excess to drip off, then dredge each piece in the flour mixture, pressing gently to adhere.

In a large skillet, heat about 1 inch of vegetable oil over medium-high heat until it reaches 350°F. Carefully add the breaded chicken pieces to the hot oil, cooking in batches if necessary to avoid overcrowding. Fry for 5-7 minutes on each side or until golden brown and cooked through. Use a meat thermometer to ensure the internal temperature reaches 165°F. Transfer to a paper towel-lined plate to drain excess oil.

While the chicken is frying, prepare the garlic mayo by combining mayonnaise, minced garlic, and lemon juice in a small bowl. Mix well and set aside.

To assemble the sandwiches, spread a generous amount of garlic mayo on the bottom half of each bun. Add a piece of crispy chicken, followed by shredded lettuce and tomato slices. Top with the other half of the bun and serve immediately.