The panino con stracchino is built around a cheese that behaves more like a condiment than a slice. Stracchino, also sold as crescenza, is a soft fresh cow's-milk cheese from Lombardy: white, rindless, faintly tangy, and loose enough to spread with the back of a knife. That spreadability is the entire premise. Where a wedge of hard cheese has to be cut to fit the bread and stacked into a layer, stracchino is smeared into the crumb so the cheese and the bread stop reading as two things and become one soft, mild, slightly sour mouthful. The discipline is to let it stay that way and not bury a delicate cheese under anything that argues with it.
The craft is moisture management and the bread it sits in. Stracchino carries a lot of water, so a soft white roll on its own goes pasty within minutes; a crusted bread with some structure, or a thin focaccia, holds the spread without dissolving under it. It is worked onto the bread at room temperature, never fridge-cold, because cold stracchino is stiff and tasteless and the whole point is the lactic tang that only comes forward when it is soft. The flavour is gentle by design, so the counter, when there is one, stays quiet: a few rocket leaves, a turn of pepper, a thread of oil. Anything stronger, a sharp salume or a vinegared vegetable, simply erases the cheese, which defeats the reason to choose it in the first place.
The named variations stay close to the Lombard and Romagnola larder. There is the stracchino and rocket build, the version folded hot into a piadina so the bread's heat just slackens the cheese, and the pairing with a mild crudo where the ham is the only added voice. Its near relations, the shredded-curd stracciatella and the soft tart squacquerone, follow their own logic and deserve their own articles rather than being crowded in here.