Jam and butter is the jam sandwich with the butter promoted from afterthought to a thick deliberate layer, and that layer is what separates it from a plain jam sandwich rather than a difference in the jam. Here the butter is spread generously, not scraped, so the sandwich has a real seam of salted fat sitting between the bread and the sweet, and the whole eating experience changes because of it. The defining note is the salt against the sugar. A bare jam sandwich is one flat sweetness; jam and butter is sweet and salt arriving together in the same bite, the fat coating the tongue and rounding the sharp fruit edge so the jam reads richer and less thin. The butter is the point of the build, not a means of getting jam onto bread.
The craft is the barrier the thick butter creates and the proportion between the two layers. Butter spread firm and to the edges is structural here above everything: it is the waterproof membrane that keeps the jam, which is wet and acidic, from soaking straight through the crumb, so a jam and butter sandwich holds together in a lunchbox where a butterless one becomes a damp red casualty by midday. The jam goes on in a restrained layer over the butter, never straight onto bread, because a thick pour slides under the top slice and squeezes out when the sandwich is pressed flat. The bread is soft plain white, because the filling has no texture and a chewy crust would be the only thing in the sandwich a bite has to fight. The two layers are kept roughly even so neither the salt nor the sweet runs away with it.
The variations move the ratio or the partner rather than the idea. More jam and less butter tips it back toward a plain sweet sandwich; clotted or whipped cream alongside turns it toward a cream-tea reading; a tarter fruit jam leans harder on the salt to balance it. Each deserves its own article rather than being crowded in here.