Prepare the chicken: Pound the chicken halves to an even 1/2-inch thickness.
Set up the breading station: In one shallow dish, mix the flour, salt, pepper, paprika, garlic powder, and onion powder. In a second dish, whisk the eggs and buttermilk together.
Dredge the chicken: Dip each chicken piece first into the flour mixture, shaking off excess. Then dip it into the wet mixture, letting excess drip off. Finally, press the chicken firmly back into the flour mixture, ensuring a thick, even coating. Set aside on a rack for 10 minutes.
Make the garlic mayo: Mix the mayonnaise, minced garlic, lemon juice, and salt in a small bowl. Set aside.
Heat the oil: Pour about 1.5 inches of vegetable oil into a deep skillet or Dutch oven. Heat the oil to 350°F (175°C).
Fry the chicken: Carefully place the coated chicken into the hot oil, ensuring not to overcrowd the pan. Fry for 4 to 6 minutes per side, until golden brown and the internal temperature reaches 165°F (74°C).
Drain: Remove the chicken and place it on a wire rack lined with paper towels to drain excess oil. Sprinkle lightly with salt immediately after removing from oil.
Toast the buns: Lightly butter the cut sides of the burger buns and toast them in a dry pan until golden brown.
Assemble the burger: Spread a generous amount of garlic mayo on both halves of the toasted bun. Place lettuce, the crispy chicken fillet, tomato slices, and pickles on the bottom bun. Top with the other bun half.
Notes
For extra crispiness, double dredge the chicken: after the first flour coating, dip it back into the wet mixture and then back into the flour mixture again.
If you prefer not to deep fry, you can air fry the chicken at 380°F (195°C) for 15-20 minutes, flipping halfway, using a light spray of cooking oil.
Brioche or potato buns work well for holding the crispy texture.