In a large bowl, combine buttermilk, salt, black pepper, garlic powder, onion powder, and lemon zest. Add the chicken breasts, ensuring they are fully submerged. Cover and refrigerate for at least 1 hour or up to overnight for best flavor.
In another bowl, mix together the flour, baking powder, and lemon pepper seasoning.
Heat about 1 inch of vegetable oil in a large skillet over medium-high heat until it reaches 350°F.
Remove the chicken from the buttermilk, allowing excess to drip off. Dredge each piece in the flour mixture, pressing lightly to adhere.
Carefully place the coated chicken in the hot oil. Fry for about 5-7 minutes on each side, or until golden brown and cooked through (internal temperature should reach 165°F). Remove and drain on paper towels.
Toast the sandwich buns lightly in the skillet or oven if desired.
Assemble the sandwiches by placing a piece of crispy chicken on the bottom half of each bun. Top with lettuce, tomato slices, and mayonnaise if using. Place the top half of the bun on each sandwich.