Richness is the whole argument of the tramezzino al mascarpone: a sweet, dense, high-fat cream cheese set inside a sweet, airy crumb, a sandwich that leans soft on soft and dares the bread to keep its shape. Mascarpone is the Lombard cream cheese, barely a cheese at all, closer to thickened cream, lactic and faintly sweet with almost no salt or tang. Spread into a crustless white triangle it meets a crumb that is also mild, also sweet, also yielding. That doubling is the defining fact. There is no sharp filling here to wake the bread up, so the mascarpone works instead as pure texture and richness, and the soft frame keeps that near-liquid cream contained. The cheese is its own bind, smooth enough to film the crumb and glue the loose triangle together; the bread is what stops a spreadable fat from having nowhere to sit.
The craft is in respecting how little structure either side brings. The loaf is a fine soft white sandwich bread, fresh that day, the crust shaved off all four sides so only the tender interior is used, and the slices kept under a damp cloth so they stay supple and never dry at the edge. The mascarpone is used cool but not fridge-cold, when it spreads as a smooth dense layer rather than tearing the crumb, and it is laid thick enough to fill the dome but not so wet that it weeps oil into the bread. That spread is the waterproofing as well as the filling: the dense cream films the inner face and seals it so a soft loaf does not turn to paste under a soft cheese. It is often given a single counter to keep the richness from going flat, a thin layer of savoury cured fish, a few herbs, or in a sweeter register a thread of honey or fruit. The mixture sits highest at the centre so the cut triangle stands with a domed middle and a thin closed edge, and a sloppy one shows fast, greasy at the cut, the bread slumping, the cream pushing out the open side.
The variations stay on the cream-rich logic and change one element. There is the savoury build that pairs the mascarpone with smoked salmon so a salty fish cuts the fat, the one that folds gorgonzola into it for a blue-veined version with bite, and the sweet build that streaks honey or soft fruit through the cheese. Each of those is the same dense cream in a soft dome adjusted by a single decision, and each deserves its own article rather than being crowded in here.