A Ganache is a glaze, icing, sauce or filling for cakes and pastries. It is made from just two ingredients chocolate and cream, blended to make a smooth velvety texture.
Ganache is a French term, the origins are unclear but they have been used desserts since approx 1850. Once made a Chocolatier or Pastry Chef will add flavour to their ganache, such as vanilla or salt, depending on the dessert they are making.
In recipes, you will also find the term Chocolate Ganache to stop it being confused from different types of cake fillings and icings.
A ganache is the 'glue' in many desserts as it can be quite versatile. It is seemingly simple to make as you 'just' heat cream and use that to melt chocolate and then slowly whip it into a firm runny ganache.
The mixture when treated right, can be left to 'set' at room temperature which is perfect for the filling between cake layers. Or you can set it in a fridge, to then roll into truffles. Or you can use it as a base for adding fruit, nuts, flavours such as vanilla or raspberry etc to have more complicated truffles or fillings between cake layers. Sometimes a ganache is used to ice a cake, usually when the cake batter is a simple clean fruity flavour and the baker wants a rich topping to set it off and doesn't want to use just cream.
There are a couple of tricks to making a ganache. Don't boil the cream, it will be too hot to pour over the chocolate. The result will be your ganache will 'split', which is when the fat solids from the chocolate and the ganache separate out - leaving it unusable.
When you start to whip the cream and chocolate together, start very slowly, so you are not actually whipping.
Really you are using a hand held egg whisk to move the chocolate around in the bowl, so the heat of the cream starts to melt it. The cream will start to take on the colour of the chocolate and when at least a third of the chocolate has melted, you can start to gentle whip the mixture.
Another reason a ganache will fail, is because you have a chocolate with very high cocoa butter solids that take skill to melt into the cream. The temperature of the cream needs to be a bit lower and you need to take a lot more time to slower blend the two ingredients. If not, your ganache will 'seize' which means the chocolate has become unstable and the ganache will become very hard and crumble if let too set - you guessed it, unusable.
And don't forget, if you add water - or water based flavours - to chocolate, it will seize then as well. Chocolate really doesn't like water, it changes the chemical composition and it will literally go dry and crumbly in front of your eyes. Technically still edible, but the chocolate won't melt in your mouth, instead it will be powdery and you will get that luscious feeling in your mouth that great quality chocolate gives you.