The Tartine Nutella is the after-school slice in its softest form: bread spread with a chocolate-hazelnut spread, handed to a child at the goûter, the four o'clock snack between lunch and dinner. The defining element is the spread itself, a sweet emulsion of cocoa and hazelnut thick enough to hold a knife mark and soft enough to go on a slice straight from the jar. That spreadability is the whole difference between this and the Tartine Chocolat, which can use a hard bar with a snap to it; here there is no snap, only an even layer. It is an afternoon slice and a child's slice, not a breakfast one, and that is the moment it owns.
The craft is in how a thick spread sits on a single slice without overwhelming it. The spread carries its own fat, so unlike jam or honey it does not need a barrier of butter underneath and is almost never given one: it goes directly on the bread and stays where it is put. The ratio is the only real discipline, because the spread is sweet enough that a layer pushed on too heavily turns the slice into mostly spread, while a thin scrape lets the bread come back through. The bread is usually a split length of fresh baguette or a soft slice of pain de mie, and softness is acceptable here in a way it is not for the bar version, since there is no hard topping that needs something to bite against. The contrast is gentle throughout: crust or crumb, then a smooth sweet layer, nothing fighting. It is best soon after it is spread, before the bread starts to draw the oil from the spread and go heavy.
The variations are the rest of this family, each a single change of topping for a single moment. Trade the spread for a hard chocolate bar and you have the Tartine Chocolat, the version with a snap; move to the morning and the slice takes jam or honey instead, the Tartine Confiture or the Tartine au Miel. Strip it back to the slice underneath and it is the plain Tartine Beurrée. The spread varies by brand and by household, but the form is constant. The Tartine Nutella sits in the open-face tradition the catalog groups under Tartine, and its particular contribution is the smoothest, sweetest version of the afternoon slice.