Prepare the chicken:

1. Season chicken generously on both sides with salt and pepper.
2. Set up a dredging station with three shallow bowls: one with flour, one with beaten eggs mixed with 2 tablespoons oil, and one with panko.
3. Dredge each chicken piece in flour (shake off excess), dip into the egg mixture, then coat thoroughly with panko. Repeat the flour, egg, and panko dip one more time for an extra-crispy crust.
4. Heat neutral oil in a cast-iron skillet to 350°F (175°C). Fry the chicken 2–3 minutes per side until deep golden and the internal temperature reaches 165°F (74°C).
5. Keep warm in a 200°F (95°C) oven if needed.

Make the curry sauce:

1. In a small saucepan over medium-low heat, melt the butter. Add the flour and stir until golden and toasty, about 2 minutes.
2. Add the curry powder, cocoa, tomato paste, Worcestershire sauce, cayenne, and black pepper. Stir and cook for 10–15 minutes, until the mixture is dry and crumbly.
3. Blend the onion, garlic, and ginger together into a paste.
4. In a separate pan, sauté the paste over medium heat until fragrant and translucent, 10–15 minutes.
5. Add the chicken stock and bring to a boil. Stir in the curry roux. Simmer, stirring often, until thickened. Season to taste with salt and sugar.

Form the rice buns:

1. Divide the cooked rice into 8 equal portions. With damp hands, shape each into a patty about 3.5 inches wide and 1/2 inch thick.
2. Brush both sides of each patty with soy sauce and a little sesame oil.
3. In a nonstick skillet over medium-high heat, sear each patty until crisp and golden on both sides, 2–3 minutes per side. Handle gently to prevent breakage.

Assemble:

1. Place one rice bun on a plate. Add a lettuce leaf and tomato slice if using, top with one piece of chicken katsu, and spoon over a generous amount of curry sauce.
2. Cap with a second rice bun. Wrap in parchment paper for stability and serve immediately.