Chocolate spread, considered as a sandwich filling, is interesting before it ever reaches bread, because it is the rare filling that arrives finished from a jar and is engineered to behave a particular way on a slice. It is a sweet, smooth, cocoa-and-fat paste, usually built on a base of sugar, vegetable oil and either cocoa or hazelnut, formulated to stay spreadable straight from the cupboard rather than setting hard like chocolate or running like syrup. That stability is the defining fact of the product and the whole reason it works between bread at all: a bar of chocolate would not spread and would not yield to a bite against soft crumb, and a chocolate sauce would soak straight through, but a stabilised spread holds a smooth, even layer that stays put.
The craft, such as the product carries on its own, is in the fat doing two jobs at once. The oil keeps the spread soft enough to drag across soft bread without tearing it, and it also acts as a partial barrier, sitting on the crumb rather than soaking in the way a water-based jam does, so a chocolate-spread sandwich stays intact far longer than a jam one and survives a lunchbox where wetter sweet fillings fail. The flavour is built to read as sweet first and chocolate second, which is why a thin layer tastes balanced and a thick one tastes cloying and slides. The version with hazelnut adds a roasted, faintly savoury note and a little graininess that cuts the sugar; the plain cocoa version is sharper and flatter. Spread thin on soft white, with or without butter underneath, it is a complete sweet filling needing nothing added, which is exactly what makes it a default rather than an occasion.
The variations are mostly what the jar gets paired with once it is treated as a component rather than a finished thing. Banana sliced into it adds water and a fruit note and shortens its lunchbox life; peanut butter on the opposite slice supplies salt and body the spread lacks; a dusting of hundreds and thousands turns it into a child's treat. The made chocolate-spread sandwich, assembled and cut for a lunchbox rather than the jar considered alone, is its own build. Each deserves its own article rather than being crowded in here.