Combine the flour, salt, black pepper, garlic powder, onion powder, paprika, and cayenne pepper in a shallow dish. In a separate shallow dish, whisk together the buttermilk and egg until smooth. Let the chicken sit in the buttermilk mixture for 10-15 minutes before coating.

Pat each chicken piece dry, dip into the buttermilk mixture, let excess drip off, then press into the seasoned flour, ensuring both sides are coated. Let the breaded chicken rest for 5 minutes.

Heat vegetable oil in a large skillet over medium-high heat until it reaches 350°F. If using no thermometer, test the oil with a pinch of flour; it should sizzle.

Add 2 chicken pieces to the hot oil and fry for 4-5 minutes per side until golden brown and internal temperature reaches 165°F. Transfer to a paper towel-lined plate. Repeat with remaining chicken.

Lightly toast the hamburger buns. Spread mayonnaise on the bottom bun, add one piece of chicken, a lettuce leaf, and cover with the top bun. Serve immediately.