The Sandwich Cancoillotte is built around a cheese that is barely solid. Cancoillotte is the Franc-Comtois cheese melted from metton curd with water and butter into a glossy, runny, almost pourable cream, eaten warm and spooned rather than sliced. It is mild and lactic with a gentle tang, and on a crusted loaf it behaves not like a filling you stack but like a sauce you spread, thick and warm and clinging to the crumb. That near-liquid state is the entire identity of the sandwich and the thing that separates it from every firm-cheese build it sits beside.
The craft is in treating a sauce as a filling. Cancoillotte is loose and warm, so it wants bread that contains rather than bread that yields: a firm crust and a close crumb that hold the cream in place instead of letting it run through. It carries its own butter, so nothing more is needed underneath, and its mildness sets the constraint, since a strong addition does not balance a gentle cheese, it drowns it. The right build is close to bare, a generous warm spread, perhaps a clove of garlic worked in as the region often does, perhaps a turn of black pepper. Portion and temperature are the whole discipline. Cancoillotte is best warm, where it stays glossy and fragrant and clings; let it cool and it stiffens toward a paste and loses the flow that is the point, and overfill the bread and the warm cream simply soaks it. It is at its best within a minute or two of being spread, while the crust is still crisp against the warm, loose cheese.
Variations are mostly aromatic, since the cheese itself does not change much. A garlic cancoillotte reads savory and rounder, a plain one cleaner and more lactic; a few slices of a regional saucisse tucked alongside add cured salt and chew against the soft cream; a turn more pepper sharpens it. Each is a recognizable adjustment of the same runny-cheese idea. It belongs with the regional cheese sandwiches the catalog groups under Baguette Fromage, and its specific contribution is a cheese kept deliberately at the edge of liquid, a warm spread the bread exists to hold together.