The golden syrup sandwich is the runniest of the British thrift sandwiches, and its defining problem is containment. Golden syrup, the pale amber inverted-sugar treacle sold by Lyle's, is not a spread that stays where it is put. It is a slow liquid, viscous but not solid, and the entire engineering of this sandwich is the question of how to keep it between two slices of bread long enough to eat. There is no second ingredient and nothing approaching a technique beyond this. The pleasure is plain bread, salted butter, and a single concentrated sweetness with a faint caramel-bitter edge, and the skill, such as it is, is stopping that sweetness from running down your wrist.
The craft is the butter, which is doing far more here than flavour. A thick layer of butter spread to the very edges of soft white bread waterproofs the crumb so the syrup does not soak straight through and out the sides, and its salt is the only thing standing between the sandwich and flat, cloying sugar. The syrup is spread thin on purpose: a generous pour is not richer, it is just messier and more sickly, and a restrained film against salted butter is the entire balance the sandwich has. The bread is soft because chew would fight a filling that offers no resistance, and the two slices are pressed together firmly so the syrup is held in a sealed thin layer rather than pooling at the bottom. Eaten promptly, before the syrup has time to migrate, it does what it is meant to do; left to sit, it becomes a study in why containment was the point.
The golden syrup sandwich sits in the broad British sweet-sandwich tradition, the childhood-and-thrift family of jam, sugar, condensed milk, and treacle on buttered bread, and within it the syrup version is the one defined by its refusal to stay put. Its near relatives are codified by which sweet liquid replaces the syrup: black treacle reads darker and more bitter, condensed milk is thicker and stays where it is laid, golden syrup with a dusting of sugar adds a crystalline crunch to the slick. Those deserve their own articles rather than being crowded in here.