You can also use pork, just be sure it is boneless and flattened.
4 boneless, skinless chicken breasts (or boneless pork) pounded flat and thin
1 cup (or more) panko crumbs
1/4 teaspoon garlic
vegetable oil for frying
1/2 cup ketchup
1 1/2 - 2 tablespoon Worcestershire Sauce
1/2 teaspoon mustard powder
1/4 cup mirin (use about 3 tablespoons of white sugar dissolved in about 2 tb+ water if you don't have mirin)
1 teaspoon Cider or white vinegar (if you used sugar instead of mirin use an extra 1/2 teaspoon vinegar)
Sesame seeds, for garnish
Combine all sauce ingredients in a small sauce pan and simmer for about 25 minutes, stirring occasionally and skimming the foam that forms from the top.
Preheat a non-stick skillet to medium heat, or pre-heat a deep fryer to the manufacturer's recommended temperature for cooking chicken.
In a flat marinating dish whisk the egg with the garlic powder and add the flattened prepared chicken, coating on all sides, allow to marinate for about 10 minutes in a fridge. Spread the panko crumbs out on a plate or platter large enough for one of the chicken breasts to fit on. Using the tongs or a fork remove one piece of chicken from the egg, allow excess to drip off and lay chicken in the panko crumb, turning to coat on all sides. Lay the breaded chicken breast on a piece of parchment paper, and repeat process on remaining pieces of chicken. Deep fry for about 6 minutes, or pan fry in vegetable oil for about 3-4 minutes per side. If unsure about how long to cook a particular piece of chicken I suggest getting a meat thermometer, they are exceptionally useful for preventing food poisoning. To serve, remove chicken from oil and lay on a cutting surface and cut into finger width strips, serve hot with tonkatsu sauce garnished with sesame seeds on the sauce and chicken.
Tonkatsu was my first adventure into Asian food, aside from a "Chinese-Canadian" style restaurant. People who truly enjoy Asian foods will laugh at me since it is basically just breaded and fried chicken with a sauce, and is Japanese not Chinese. I know these things, but I will be the first to admit that I am not very well versed in Asian food. At all. But I do at least know what I like!
A key feature of this recipe is the chicken (or pork), in order for it to have the right texture the chicken must be pounded flat and thin. If you have a fully stocked kitchen you might have a mallet that will do this job excellently. If not I recommend wrapping an empty wine bottle up in some plastic wrap and using that, or a rolling pin, I have a lovely marble rolling pin that has some excellent heft to it, which is good when you want to pound something to be about 1/2 inch thick. Or really I have seen my mother just put the chicken in a plastic ziplock and pound it with the bottom of her fist. As long as the chicken (or pork) gets thin and flat, it'll all work out. The other key feature to get this dish to work out right is getting the right bread crumb, you need panko bread crumbs and only they will do. Luckily panko is available at most grocery stores now, or Bulk Barn, or really just check out your local Asian market/store. Oh and While you are there pick up some mirin, it is a Japanese sweetener, that works excellently in a lot of sauces and dips. Plus since it is a liquid it is much more miscible in the sauce, and there wont be any undissolved grains of sugar in the final product. You can use sugar, but I would try and dissolve it into water before adding it to the sauce.