Trim away excess fat, connective tissue, and tendons from the chicken, then cut into chunks.

Add the chicken to a food processor with the garlic powder, paprika, salt, white pepper, sugar, and chicken bouillon. Pulse until the chicken is very finely ground with an almost paste-like consistency.

Divide the chicken mixture into 6 equal portions. With clean or gloved hands, form each portion into a thin patty about 4–5 inches wide. Arrange on a parchment-lined baking sheet.

Freeze the patties for about 1 hour, or until firm enough to easily handle without becoming floppy.

About 10 minutes before removing the patties from the freezer, pour approximately 1 inch of neutral oil into a large, heavy skillet and heat to 350°F.

In a wide bowl or pie dish, whisk together the flour, cornstarch, garlic powder, paprika, salt, white pepper, sugar, chicken bouillon, and black pepper.

Remove the firm patties from the freezer. Working carefully, thoroughly dredge each patty in the dry flour mixture, coating both sides and around the edges. Return them to the parchment-lined pan.

Pour 1 cup cold club soda into the remaining flour mixture and whisk until smooth. Add additional club soda a splash at a time, up to approximately ¼ cup more, until the batter reaches the consistency of heavy cream. It should be fluid and easily coat the chicken without being thick like pancake batter or thin and watery.

Working one at a time, place a dredged patty into the wet batter. Use two forks to carefully flip it, making sure the entire patty and edges are coated.

Lift the patty from the batter, allow the excess to drip briefly, then carefully transfer it to the 350°F oil.

Fry 2–3 patties at a time, without overcrowding the skillet. Cook for a few minutes per side, until the coating is golden brown and crispy and the chicken reaches an internal temperature of 165°F.

Transfer cooked patties to a wire cooling rack so excess oil can drain without making the bottoms soggy. Bring the oil back to 350°F between batches and repeat with the remaining patties.

Lightly butter the cut sides of each hamburger bun. Toast in a clean skillet over low heat until lightly golden, or arrange cut-side up on a baking sheet and toast in a 350°F oven.

Spread mayonnaise over the top and bottom buns. Add a crispy chicken patty, pile generously with finely shredded iceberg lettuce, and finish with the top bun. Serve immediately.