Cut slits along the fat of the pork loins (this prevents pork from curling as it fries). Tenderize each piece of pork with a meat mallet or with the back of your knife until about 1 cm thick.
Evenly season pork loins with salt, black pepper and garlic powder on both sides. Place flour in a large shallow bowl, beaten eggs in a medium bowl and panko in a third bowl. Evenly dredge each pork loin first in flour, then in egg and lastly panko - ensuring no bald spots.
In a heavy bottomed pot, heat oil over medium heat or 350 F. Once oil is hot, deep fry pork on each side until golden brown, 2-3 minutes per side and until the pork reaches and internal temperature of 145 F or juices run clear.
Remove pork with a slotted spoon and rest the pork on a wire rack or large plate or baking sheet lined with paper towels to excess drip the oil. The residual oil will make the crispy pork soggy.
In a bowl, combine grated cabbage, salt, rice vinegar and Japanese mayo. Mix well.
Butter two slices of bread. Layer a piece of pork katsu on top, drizzle Tonkatsu sauce over top, followed by equal portion of cabbage slaw.
Close the sandwich with a slice of bread. Slice off the crusts (optional) and then slice sandwich in half with a bread knife. Enjoy!