Mascarpone is a fresh, very rich cow's-cream cheese from Lombardy. Acid sets it. It is not aged, and it has no rind.
It is closer to soft butter than to a firm block. Use it in sweets, above all tiramisu, and in a few rich sauces. It will not crumble, and it will not keep like a hard cheese.
History
Mascarpone is a cheese of the dairy country of Lombardy, made from cream rather than from a lean curd. The region already specialized in rich cow's-milk products. A soft acid-set cream was a local fresh cheese before it was a worldwide dessert ingredient. Exact origin stories that name one town and one year are less solid than the regional fact. Lombardy is the home.
Tiramisu made the name international. That dessert, built from mascarpone, egg, sugar, and coffee-soaked biscuits, spread from Italy in the late twentieth century until the cheese became a supermarket item in many countries. The dessert is modern. The cheese is older than the dessert. Buying mascarpone only for tiramisu is common, and it is a narrow use of a simple product.
How it is made
Cream is warmed and coagulated with acid, traditionally something like tartaric acid or a cultured whey, then drained until it is thick. There is no long brine, no blue mold, and no cave. The cheese is finished when the texture is like a dense, spreadable cream. It is packed cold and sold fresh.
Fat and drainage decide the quality you can taste. A good mascarpone stands up in a spoon and still feels silky. A thin, sour, or grainy tub was under-drained, over-acidified, or kept too long. It is not a cheese you can age into something better. Producers outside Lombardy now make the style. The method is the style. A protected name is not what this page is selling.
Taste
The taste is sweet cream, with a light tang and almost no salt by comparison with a cheddar or a Brie. It should taste clean and rich. If it tastes buttery in a fresh way, that is the cheese. If it tastes oily, cheesy, or sharply sour, the tub is off.
Cold mascarpone tastes muted. Let it soften before you judge a plain spoonful, and before you beat it into a dessert. Warmth makes it taste sweeter and fuller. It will also melt into liquid if you leave it in a hot room. Soften it. Do not abandon it.
In the kitchen
The main use is sweet. Beat it with eggs and sugar, or simply with sugar, for tiramisu. Fold it with fruit, or use it in place of some of the cream in a cake. In savory cooking, a spoonful can enrich a risotto or a mushroom sauce at the end. Keep the heat low so the cream does not split into fat and watery curds.
Do not grate it, fry it, or put it on a burger. Do not swap it for cream cheese in a baked cheesecake without expecting a richer, looser, less tangy cake. Cream cheese is tangier and firmer. Crème fraîche is sourer and thinner. Clotted cream is closer in richness on a scone, and it is still not the same in tiramisu. For tiramisu, mascarpone is the ingredient, not a suggestion.
Around the world
Lombardy is the source of the style. Tiramisu carried it into restaurants and home kitchens on every continent. Many people now meet mascarpone first as a dessert ingredient in a plastic tub, with no mention of a region. That is a fair meeting. The cheese is a fresh dairy product, not a wheel with a stamp.
A tub made outside Italy can be excellent if the cream was good and the texture is thick. A tub made in Lombardy can be dull if it was thin and old. Use it soon. When a recipe from another cuisine says cream cheese and you use mascarpone, reduce the richness elsewhere or accept a heavier dish. When an Italian dessert says mascarpone, cream cheese will taste too sharp.