The Tartine Chocolat is the afternoon slice, the four o'clock goûter handed to a child coming home from school: a length of bread and chocolate, eaten standing in the kitchen, not at the breakfast table. The defining choice is the chocolate's form. This is the version where the chocolate can be a broken square or bar laid against the bread rather than only a spread, and the bar is what sets it apart from the Tartine Nutella, which uses a soft chocolate-hazelnut spread instead. The moment matters as much as the topping: this slice belongs to the gap between lunch and dinner, not to the morning the buttered tartines own.
The craft is in the contrast between a hard topping and a soft one, and the two builds behave differently. The bar version is squares of dark chocolate pushed into a split length of fresh baguette, sometimes still slightly warm so the chocolate goes just soft at the edges where it meets the crumb: the bite is crust, then snap, then the chocolate yielding against soft bread, three textures in one mouthful. A film of butter under the chocolate is common and bridges the snap to the wheat. A spreadable plain chocolate is the smoother alternative, even across the slice with no snap to it, and reads as one continuous layer rather than a contrast. The bread wants enough crust to stand up to the hard chocolate, since soft bread against a hard bar gives nothing to bite against. It is best soon after it is built, while the bread still has its bite and the chocolate has not fully hardened back.
The variations are the surrounding family, each a single swap for a single time of day. Trade the bar for a chocolate-hazelnut spread and you have the Tartine Nutella, the softer after-school version; move back to the morning and the slice takes jam or honey instead, the Tartine Confiture or the Tartine au Miel. Strip it to the slice underneath and it is the plain Tartine Beurrée. The chocolate varies from dark to milk by who is eating it. The Tartine Chocolat sits in the open-face tradition the catalog groups under Tartine, and its particular contribution is the snap of a hard bar against soft bread at four in the afternoon.