A standard katsu sando asks one cut of pork to do everything: hold a crisp crust, stay juicy, and survive being bitten through two slices of bread without dragging the whole slab out in one tug. The mille-feuille version answers a different question. Instead of a single thick loin, it stacks several thin slices of pork on top of each other, sometimes seasoning or lightly flouring between the layers, then breads and fries the whole stack as one block. The name borrows from the French pastry of a thousand leaves, and the comparison is honest: the cut surface shows fine horizontal striations rather than one dense mass of meat.
The reason to bother is texture. A thick tonkatsu cutlet can fight back; the mille-feuille construction gives you the same height and the same crackling panko shell, but the interior shears apart in soft, distinct sheets when you bite, so the resistance is gentler and more even. Each thin slice has its own faint edge of rendered fat, which means the richness is distributed through the stack rather than concentrated in one band. The bread is the usual soft shokupan, crusts off, and the sauce is the usual tonkatsu lacquer, dark and sweet and slightly tangy, sometimes with a stripe of karashi mustard. The bind matters more here than in most cutlet sando: the layers have to fuse during frying so the stack reads as one cutlet, not a deck of cards that fans apart and spills. A good one is sliced cleanly so the leaf structure is visible end-on and the panko stays shattering-crisp against the soft bread; a sloppy one has layers that slide loose, a soggy underside where sauce has pooled, or a stack so thick it still bullies the bread the way a single loin would.
Variation tends to track the meat and the seasoning between layers. Some kitchens tuck shiso leaves, a thin smear of plum paste, cheese, or nori between the slices so the flavor changes as you eat down through the stack. Pork is the default, but the technique transfers to chicken and to thin beef. Cabbage, shredded fine, sometimes rides along inside the sando for crunch and to lighten the fat. The parent katsu sando, with its single-slab loin and its long argument about loin versus fillet versus the perfectly even cross-section, has enough history and enough partisans that it deserves its own article rather than being crowded in here.