Katsu sauce: in a medium bowl, combine all ingredients and whisk to fully incorporate.

Store in the refrigerator for up to two weeks.

Tonkatsu preparation: Cut your pork tenderloin into a shape around the size of your bun. For me, this was around 5 ounces. 

Butterfly each piece by cutting the tenderloin down the middle about two-thirds of the way through. Don't cut all the way. This will allow you to "open" the meat like a book.

Place your butterflied meat on a cutting board and cover it with a piece of plastic wrap. Using a meat mallet or something heavy like a cast iron skillet, pound the plastic wrap covered meat until each piece is about a quarter of an inch thick. 

Set up a breading station with three bowls or plates. 

The first bowl should have all-purpose flour in it. The second bowl will contain your egg and buttermilk. Mix to combine. 

The third bowl will contain your panko breadcrumbs along with salt, pepper, onion powder, garlic powder, and paprika. Stir well.

Dunk and coat each piece of tenderloin in flour and then in the egg mixture, flipping a few times in both bowls to fully coat. 

Then bread each egg-coated tenderloin and coat it in the bowl with the seasoned Panko breadcrumbs. Make sure to press the breadcrumbs into the pork to fully coat each piece. 

Once each piece is fully coated, move to a plate or sheet pan to rest.

In a pot or pan over medium-high heat, add 2 inches of peanut or other vegetable oil for frying. When the oil has reached 350 F degrees you are ready to fry. 

Add each tenderloin or work in batches if they don't both fit in the pan without overlapping. 

Cook each tenderloin on the first side for 3 to 4 minutes. Flip and cook for an additional 3 to 4 minutes. At this point, you can check for doneness with a meat thermometer. 145 is done for pork, but some people will prefer it more than that. I personally pull mine out around 140 because it will continue to rise in temperature for a few minutes after being removed from the oil. 

Transfer to a paper towel-lined plate to let some of the excess oil drip off. 

Tonkatsu sandwich assembly: place two slices of bread down on a cutting board or clean surface. Add katsu sauce to both slices. 

Top the katsu sauce with a big handful (or half) of the shredded cabbage. Top the cabbage with a fried pork cutlet. 

Drizzle katsu sauce on top of the pork cutlet. Optionally add mayonnaise and/or Dijon mustard to the other slices of bread and use those slices to complete the sandwich. 

For a traditional presentation, cut the crusts off the bread and slice the sandwich in half.

Serve and enjoy.