The samosa sandwich is carbohydrate folded into carbohydrate, and the defining component is the pastry shell, not what is inside it. A whole samosa, the deep-fried triangular pastry packed with spiced potato and peas or with minced meat, is put between slices of soft buttered bread, and the result is a sandwich that already contains a complete fried snack. The lead is that crisp, blistered, oil-fried shell. Take a spiced potato filling and spoon it into bread and you have a different, softer thing entirely; keep the samosa whole and intact and the sandwich is built around a brittle case that has to survive being pressed between two slices, which is the whole engineering problem and the whole appeal.
The craft is protecting the shell and managing the spice. The samosa is best slightly warm and is added at the last moment, because the soft bread will steam the pastry limp the longer it sits, and a samosa sandwich whose case has gone soft has lost the only textural contrast it had. It is often lightly crushed under the top slice so the shell cracks and the sandwich bites cleanly rather than skidding apart, the shards of pastry then trapped against the bread the way a crisp sandwich traps its crisps. The filling brings the heat, an aromatic, cumin-and-chilli potato or a spiced mince, so a cooling element does the structural work that butter alone cannot: a smear of mango chutney for sweet acid, or a spoon of mint or tamarind chutney worked against the bread to carry moisture back under control. The bread is soft and plain because the samosa is already loud in both flavour and texture and a chewy crust would only fight a filling that brings its own crunch.
The variations track the snack rather than the sandwich. The potato-and-pea samosa is the vegetarian default; the lamb or chicken keema samosa runs spiced mince; the chutney swapped between mango, mint, and tamarind retunes the cool counter each time. Its near relatives are the onion bhaji and pakora sandwiches, the same impulse of putting a fried fritter into bread. Each deserves its own article rather than being crowded in here.