Prepare the sauce: in a mixing bowl combine 3/4 cup mayonnaise, 1 tablespoon ketchup, 1 teaspoon Dijon mustard, 1 teaspoon dill relish (or finely chopped pickles), 1/2 teaspoon Worcestershire sauce, 1/2 teaspoon garlic powder, 1/2 teaspoon onion powder, and 1/2 teaspoon paprika; season with salt and pepper to taste, mix until smooth, cover and refrigerate at least 30 minutes.
Combine the dry breading: in a shallow dish whisk together 1 cup all-purpose flour, 1 tablespoon paprika, 1 teaspoon garlic powder, 1 teaspoon onion powder, 1 teaspoon salt, and 1/2 teaspoon black pepper.
Make the wet dip: in a second shallow dish whisk 1 cup buttermilk with 2 eggs until blended.
Coat the chicken: dip each trimmed chicken cutlet into the buttermilk-egg mixture, then dredge thoroughly in the seasoned flour, pressing gently so the flour adheres; shake off excess.
Heat oil: pour enough vegetable oil into a large skillet to reach about 1/2 inch depth and heat over medium-high until it reaches about 350°F (use an instant-read thermometer).
Fry the cutlets: carefully add the breaded cutlets to the hot oil (do not overcrowd) and fry 4 to 5 minutes per side until golden brown and the internal temperature reaches 165°F; adjust heat as needed to maintain 325–350°F.
Drain the chicken: transfer fried cutlets to a wire rack set over a sheet pan or to paper towels to drain briefly.
Toast buns: toast the 4 hamburger buns until lightly golden.
Assemble sandwiches: spread about 1 tablespoon BK Royal Sauce or mayonnaise on the bottom of each bun, place a fried chicken cutlet on top, add a tomato slice and two lettuce leaves, then cap with the top bun and serve immediately.
