The Tartine Beurrée is the baseline against which every other tartine is measured: one slice of bread and a layer of butter, eaten at breakfast and almost nowhere else. There is no second slice and no third ingredient. A length of yesterday's baguette, or a cut from a round of pain de campagne, is laid flat, spread edge to edge with cold butter, and eaten with a bowl of coffee or chocolate beside it. Everything the French sandwich shelf does with jam, honey, chocolate, and cheese starts from this slice and adds one thing to it. Strip those additions away and what remains is the morning tartine in its plainest form.
What makes it work is the ratio, and the ratio is unforgiving because there is nothing to hide behind. The butter has to be thick enough to read as butter rather than as a film, which in practice means a layer you can see the knife marks in, pushed all the way to the crust so no dry corner is left. The bread carries the whole structure, so its texture is the entire experience: a baguette gone slightly firm overnight gives a crust that resists and a crumb that yields, which is the contrast the slice is built on. A demi-sel butter does most of the seasoning work, the salt reading against the wheat without anything else in the way. Spread the butter too thin and the tartine is dry; cut the bread fresh and soft and there is no resistance to push against. The slice is best within a few minutes of being buttered, before the bread surrenders its bite.
The variations are the rest of this family, each one this slice plus a single topping for a single moment. Add jam over the butter and you get the Tartine Beurre-Confiture, the canonical petit-déjeuner. Reach for honey instead and it is the Tartine au Miel; for the afternoon goûter it takes chocolate or a chocolate-hazelnut spread. None of those crowd in here, because the buttered slice is the thing they all have in common and the one worth understanding on its own. The Tartine Beurrée belongs to the open-face tradition the catalog groups under Tartine, and its specific role is to be the slice everything else is measured from.