Place the chicken pieces in a bowl and cover them with buttermilk, salt, pepper, paprika, and cayenne pepper. Marinate in the refrigerator for at least 2 hours or up to 8 hours.
In a shallow dish, whisk together the flour and cornstarch.
Remove chicken from the buttermilk, letting excess drip off, but do not wipe dry.
Dredge each piece thoroughly in the flour mixture, pressing the coating onto the chicken. For extra crispiness, dip the coated chicken back into the buttermilk briefly, then dredge again in the flour mixture, ensuring a thick, craggy coating.
Heat about 2 inches of vegetable oil in a deep skillet to 350°F (175°C).
Carefully place chicken in the hot oil, ensuring not to overcrowd the pan. Fry for 5-7 minutes per side, until golden brown and the internal temperature reaches 165°F (74°C). Remove and drain on a wire rack.
While the chicken cooks, prepare the garlic mayo: Mix the mayonnaise, minced garlic, lemon juice, and salt in a small bowl. Stir until combined.
Toast the cut sides of the burger buns lightly in a dry pan or under the broiler.
To assemble the burger, spread a generous layer of garlic mayo on both halves of the bun.
Place lettuce and pickles on the bottom bun. Top with a crispy chicken patty and cheese (if using). Place the top bun on the chicken.
Notes
To prevent a soggy bun, lightly toast the buns and spread a thin layer of garlic mayo on both sides before adding wet toppings.
For an air fryer method, spray the coated chicken lightly with oil and cook at 380°F (195°C) for 15-18 minutes, flipping halfway through.
Use fresh garlic for the best flavor in the mayonnaise.