Ganache is a word that gets tossed around a lot in cookbooks and on dessert menus, and while it is easy to figure out that it has something to do with chocolate, what makes this particular kind of chocolate a ganache is often a little less clear. Ganache is a mixture of melted chocolate and cream that can be used as a glaze for pastries and desserts, a filling for cakes and for making chocolate truffles. The basic ratio used when making ganache is two parts chocolate (usually a semisweet/bittersweet chocolate) to one part cream. The mixture is heated just until the chocolate melts, then it can be poured directly on to the dessert you want to glaze with it. It creates a very smooth, glossy glaze with a luxurious mouthfeel, as the cream makes the chocolate much smoother than it would be on its own.
The ganache is used as a glaze while it is still warm to make it pourable, as it thickens up when cooled to room temperature. Cooled ganache is firm enough to handle, but it can be shaped and still melts on the tongue very easily. Cooled ganache is used to make classic chocolate truffles and can be used as the filling for other chocolate confections. Cooled ganache can also be whipped to a mousse-like consistency by beating it with a mixer for a few minutes until it becomes light and fluffy. This mousse-like ganache is great for filling cakes and other pastries.
Ganache recipes can vary slightly in the ratio of chocolate to cream, depending on the intended use of the ganache and the preference of the author of a particular recipe. Some recipes will even use butter in place of the cream, for a different texture to the ganache (although this is typically something I use to coat candies, not to fill chocolates). While a classic ganache is made with a bittersweet chocolate, ganache can be made with other types of chocolate, such as white chocolate and milk chocolate. Ganache is delicious on its own, but it can also be flavored to make different flavored truffle fillings.