The treacle sandwich is the darkest and least sentimental of the sweet sandwiches. Black treacle is spread on soft buttered bread, and that is the entire build, but the choice of treacle rather than jam or syrup is the whole character. Black treacle is bitter as much as it is sweet, with a burnt, almost mineral edge that golden syrup does not have, so the sandwich carries a contradiction in one bite: sticky and sweet at the front, bitter and faintly smoky at the back. It is thrift food in the plainest sense, a way to make bread and butter into something with a strong flavour out of one cheap, long-keeping tin.
The craft is almost entirely the butter and the spreading. The butter is structural, not a trimming: spread to the edges it waterproofs the crumb so the treacle does not soak straight through to a soggy patch, and its salt is what stops a bitter syrup reading as flat and one-noted. The treacle is spread thin, because it is intense and very sticky and a thick layer slides out under the lightest press and clags the mouth shut. The bread is soft and plain, since a treacle this assertive needs no help from the loaf and a chewy crust would only fight a filling that has no texture of its own. Eaten soon after it is made, before the treacle has a chance to creep through the butter, it holds together as a clean sandwich rather than a smear.
The variations stay inside the dark, sticky, soft frame and mostly soften the bitterness. A mix of black treacle with golden syrup pulls it sweeter and less burnt; a layer over a sweet bread such as bara brith leans it toward cake; a squeeze of lemon cuts the sweetness the way it does on a treacle tart. It sits alongside the other thrift sweets, the sugar sandwich and the jam piece, as the same instinct met with a darker tin. Each deserves its own article rather than being crowded in here.