The panino con mozzarella di Gioia del Colle is the cow's-milk answer to the buffalo sandwich, and the difference in milk is the whole difference in the sandwich. The protected mozzarella of Gioia del Colle, in the Murgia of Puglia, is made from cow's milk and comes out firmer, more compact, and drier than the wet Campanian bufala, with a sweeter, gentler, more buttery flavour and a lactic tang that is present but restrained. Because it holds together, it can be sliced cleanly and behaves on bread rather than flooding it. That structural calm is the point: it is a mozzarella sandwich you can build with some composure, the cheese the single voice but a steadier one.
The craft follows from a curd that cooperates. The Gioia del Colle is sliced rather than torn, because it is firm enough to cut and keep its shape, and it sheds far less whey, so the bread does not have to be a bulwark against liquid the way it does with bufala. A plain loaf with a light crust is enough, the assembly less frantic because the cheese is not racing to soak the crumb. The dressing stays spare on purpose, a thread of Pugliese olive oil, a little salt, perhaps a leaf, because the milk's own buttery sweetness is the statement and an assertive sauce would flatten a flavour that is gentle by design. It is eaten cool but not fridge-hard, when the curd is supple and the milk reads sweetest.
The variations are local pairings on a cheese that holds its form. There is the version with Pugliese tomato and oil, the reading with capocollo or another southern cured meat, and the grilled treatment that turns the firmer curd into a clean melt. The wet Campanian buffalo mozzarella is a distinct sandwich of its own, a different milk and a different problem. Each of these deserves its own article rather than being crowded in here.