Ganache is chocolate and cream: the fat of the chocolate and the water of the cream form a stable emulsion. Depending on its temperature it can be a glaze, a filling or a frosting. The typical problem is a greasy, split ganache, caused by overheating or cream that doesn’t have enough fat.

Ingredients and what each one does

Dark chocolate gives flavour and structure: cocoa butter sets as it cools, which is why ganache firms up.
Cream supplies the water and fat that dissolve the chocolate. Use at least 30% fat, otherwise the emulsion splits. Here the ratio is 1:1 by weight, which gives a thick, glossy glaze.
A little cold butter at the end adds shine and smoothness.

How to make it

Chop the chocolate finely: smaller pieces melt faster and more evenly.
Heat the cream until hot, about 80–90 °C, when foam appears at the edge. Don’t boil it hard: boiling off water shifts the ratio.
Pour over the chocolate and leave it for 2 minutes untouched while the heat melts it from inside.
Stir from the centre in small circles until smooth and glossy. Don’t whisk with a mixer: air makes it paler.
Cool to the consistency you need. Temperature controls everything: at 34–36 °C ganache is runny and gives thin drips; at around 28 °C it is thicker. Test it on a glass. For pouring over a cake, around 32 °C works well.
Chill the cake first, even in the freezer, because a cold surface sets the ganache quickly and stops the drips. They run for roughly 2 minutes and can be tidied with a hair dryer. Cooled ganache whipped to a pale colour becomes a filling.

What can go wrong

Split, shiny with oil. Overheated, or too much fat. Stir in a spoon of warm cream or milk; the emulsion often comes back together.
Matt and grainy. The chocolate didn’t melt fully, or the cream wasn’t hot enough.
White streaks. Cocoa butter that surfaced after sharp temperature changes. It doesn’t affect the taste.
Too many drips, or too thick. Temperature: warm it up or let it cool.

Substitutions

Milk and white chocolate have more sugar and milk solids and less cocoa, so they need less cream to set; test a small batch. Cream with lower fat won’t work.