Prepare the Pork: Make a few shallow cuts through the fat cap of the pork chop to prevent it from curling. Pound the meat gently to an even thickness. Season generously with salt and pepper.

Bread the Katsu: Dredge the pork in flour, dip it completely into the beaten egg, and press it firmly into the Panko breadcrumbs to create a dense, thick crust.

Fry the Katsu: Heat 1 inch of vegetable oil to 340°F (170°C). Fry the breaded pork chop for 5 to 6 minutes, flipping once, until deeply golden brown and fully cooked through. Let it rest on a wire rack for 3 minutes.

Prepare the Bread: Trim the crusts off the soft Japanese milk bread. Mix the softened butter with the Karashi mustard and spread it evenly on the inside of both slices of bread. This acts as a barrier to keep the bread from getting soggy.

Assemble and Slice: Slather the resting Tonkatsu generously on both sides with the tangy Tonkatsu sauce. Place the saucy pork between the buttered bread slices. Press down very gently. Use a sharp knife to cut the sandwich in half and serve immediately.