Begin by preparing the chicken. Place the chicken thighs between two sheets of plastic wrap and pound them to an even thickness of about 1/2 inch using a meat mallet or rolling pin.

Set up a breading station: In one shallow dish, combine the flour, salt, black pepper, garlic powder, onion powder, and cayenne pepper. In a second dish, beat the eggs. In a third dish, place the panko breadcrumbs.

Dredge each chicken thigh first in the flour mixture, shaking off any excess. Then dip it in the beaten eggs, allowing any excess to drip off, and finally coat it in the panko breadcrumbs, pressing gently to adhere.

In a large skillet, heat about 1/2 inch of vegetable oil over medium heat until hot (around 350°F). Carefully add the breaded chicken thighs to the skillet, cooking in batches if necessary to avoid overcrowding. Fry for about 4-5 minutes per side, or until golden brown and cooked through. Remove and drain on paper towels.

While the chicken is frying, prepare the sauce by mixing the mayonnaise, ketchup, and soy sauce in a small bowl.

To assemble the burgers, spread a generous amount of the sauce on the bottom half of each hamburger bun. Place a fried chicken thigh on top, add a handful of shredded cabbage, and cover with the top half of the bun.

Serve immediately and enjoy your crispy chicken katsu burger!