The tattie scone roll is carbohydrate folded inside carbohydrate, and the thing that makes it itself is the texture of the scone. A tattie scone, the Scottish potato scone, is mashed potato bound with a little flour and butter and griddled flat into a soft, pliable round, dense and faintly starchy with a lightly browned skin. The defining fact of this roll is that softness. It is not bread inside bread in any crisp sense: it is a yielding, almost custardy potato layer slid into a soft morning roll, so the whole sandwich is one note of gentle, warm, doughy starch with very little contrast deliberately built in. The pleasure is the comfort of that uniformity, not the tension most sandwiches are designed around.
The craft is the heating of the scone and the choice of roll around a filling that brings no moisture and no salt of its own. A tattie scone is best warmed through on a griddle or under heat until the skin takes a little colour and the inside is soft and hot, because a cold scone is heavy and pasty and the warmth is what makes the starch read as comforting rather than stodgy. It goes into a soft floured roll, the same absorbent carrier the whole Scottish breakfast shelf runs on, and butter is close to essential here, less for richness than because two plain starches with nothing wet between them need the fat as the only thing carrying flavour and easing a dry bite. A stripe of brown or red sauce, applied inside, is the one sharp counter the roll usually gets, cutting a filling that is otherwise entirely soft and mild. The scone is the statement and the roll frames it rather than competing.
The variations stay inside the soft breakfast-roll frame and mostly add a single counter to the same starchy constant. A rasher of bacon brings salt and a brittle crunch the plain roll lacks. A fried egg lays a soft yolk over the scone and binds the roll. A slice of Lorne sausage or black pudding turns it into a fuller Scottish breakfast in one hand, and the scone served alongside on a plate rather than in a roll is the same component read as a side. Each deserves its own article rather than being crowded in here.