In a large mixing bowl, combine buttermilk, garlic powder, onion powder, paprika, salt, and black pepper. Whisk until well blended.
Add the chicken breasts to the buttermilk mixture, ensuring they are fully coated. Cover and refrigerate for at least 15 minutes (or up to overnight).
In a shallow dish, place the flour and season with additional salt and pepper, if desired. Mix well.
Heat the vegetable oil in a frying pan over medium-high heat until hot (about 350°F).
Remove each chicken breast from the buttermilk mixture, allowing excess to drip off. Dredge each breast in the seasoned flour, ensuring an even coating.
Carefully place the coated chicken breasts in the hot oil. Fry for 6-7 minutes on each side, or until golden brown and cooked through (internal temperature should reach 165°F). You may need to do this in batches.
Once cooked, transfer the chicken to a plate lined with paper towels to absorb excess oil.
In a small bowl, mix mayonnaise, Dijon mustard, and lemon juice. Spread this sauce on the insides of the hamburger buns.
Assemble the sandwich by placing a fried chicken breast on each bun, topped with a slice of tomato and a lettuce leaf.
Close the sandwich and serve immediately.