The Mini-Sandwich is defined by its size and by where it gets eaten. It is the sandwich scaled down to a few bites, made for the catering tray rather than the lunch counter: a reception, an office gathering, a buffet where the food is held in one hand and a glass in the other. The format is the whole identity. Shrinking a sandwich to a bite changes what it has to do, because it is now one item among many on a plate rather than a meal on its own.
The construction is built around that constraint. The bread is most often a fine-crumbed pain de mie with the crusts trimmed, or a short length of soft baguette, because at this scale a hard crust shatters and scatters rather than yielding cleanly. The filling is a single clear idea, not a stack: ham and butter, a herbed cream cheese, smoked salmon, a thin spread of pâté with a sliver of cornichon. Multiple competing layers do not survive being read in one or two bites, so the good versions commit to one flavor and execute it precisely. The sandwich is then cut into small rectangles or triangles and arranged in rows.
The craft is mostly about holding texture across time, since these are almost always made ahead and set out rather than served to order. The enemy is the bread drying at the cut edges or going damp from a wet filling under refrigeration. A traiteur who understands this uses a tight crumb, spreads butter as a moisture barrier between bread and filling, keeps the filling on the drier side, and covers the tray until the last moment. Done well the bite stays soft and distinct; done poorly it curls at the edges and tastes of the fridge. At this size there is no second component to hide a stale one behind, so the margin for error is narrow.
Variations are mostly filling choices within the same small frame: a savory mousse, a single charcuterie, a cheese with something sweet, an egg-and-cress in the tea-room register. Each is a one-idea bite scaled to the tray. The broader catering tradition it belongs to, the platter-assembled, eaten-standing-up format, is the Sandwich Traiteur & Boulangerie, of which the mini is the smallest expression.