The Pain au Chocolat Sandwich is the rare entry in the French repertoire where the bread is a pastry and the filling is dessert. A pain au chocolat is laminated dough, the same butter-layered build as a croissant, wrapped around two batons of dark chocolate and baked until the outside shatters and the inside stays soft and faintly molten at the chocolate. Splitting one and filling it produces a sandwich that no savoury loaf could stand in for: a baguette gives structure and a neutral base, while this gives crisp flake, butter, and a built-in seam of chocolate that the filling is there to play against, not to overcome.
The pastry sets the rules, and they are sweet ones. The most common filling is a scoop of ice cream pressed between the warm split halves, the heat softening one side while the cold holds the other, the chocolate sitting in the middle as a third texture. Whipped cream, a fruit compote, a chocolate-hazelnut spread, or sliced banana work on the same principle: something cool or soft set against the warm laminated crisp. A heavy or savoury filling has no business here; it would fight both the butter and the chocolate. The eating is brief and a little unstable by design. The pastry compresses and flakes as it is bitten, the filling escapes at the edges, and the whole thing is gone before the crisp has any chance to soften, which is the point of making it from a pastry rather than a bread.
The variations all stay in the dessert register. The ice-cream version is the type case; the others swap in cream, fruit, or spread while keeping the warm-pastry-against-cool-filling logic intact. It is the sweet outlier in the family of French sandwiches built on a bread other than the baguette, gathered under Pain Garni & Non-Baguette Breads, and within that set it is the entry where the loaf has been replaced by a chocolate pastry and the sandwich has quietly become pudding.