The panino con mozzarella di bufala is a sandwich organised around a problem: the cheese will not stay put. Campanian buffalo mozzarella is wet by nature, a soft, elastic curd that holds milky whey inside it and weeps the moment it is torn, so the defining sensory event is the spill, the cool flood of buffalo milk released into the bread on the first bite. The flavour is lactic, faintly sour, richer and more animal than cow's milk, and the texture is yielding rather than sliceable. On bread it is the single voice, but it is a difficult one to carry, and the whole craft of the sandwich is about managing how much liquid the bread can take before it gives way.
The craft is moisture control above everything. The bufala is drained and torn rather than cleanly sliced, by hand into rough pieces, both because it does not cut neatly and because tearing lets surplus whey run off before it reaches the crumb. The bread is chosen with a real crust and an open, sturdy interior, not a soft white roll, because it has to stand against a filling that is actively wet, and assembly is fast and immediate: a buffalo mozzarella sandwich built ahead and left is a soaked one. The dressing is minimal and deliberate, a thread of good oil, a little salt, perhaps a basil leaf, because the cheese is loud enough and anything else would only add more liquid to a structure already at its limit.
The variations are about what one quiet partner is allowed to join the cheese without overwhelming it. There is the caprese reading with tomato and basil, the version with a few folds of prosciutto crudo, and the grilled and pressed treatments that cook the curd into threads rather than leaving it raw and weeping. The firmer Puglian cow's-milk mozzarella is a distinct sandwich of its own. Each of these deserves its own article rather than being crowded in here.