Ganache
Ganache is an emulsion of chocolate and liquid, most often cream. For all its apparent simplicity it is precisely that: cocoa butter fat dispersed as fine droplets through the water phase of the cream.
The ratio sets the purpose. Roughly one to one by weight gives a soft ganache for fillings and layers. Two parts chocolate to one gives a firm one, suitable for cutting and for moulded bonbons. One part chocolate to two of cream gives a pouring ganache for sauces and glazing.
The type of chocolate matters too: milk and white contain less cocoa butter and more sugar, so they take noticeably less cream than dark.
Technique decides everything. Hot cream goes into the chocolate in stages, starting with a small addition, stirred from the centre — the emulsion builds outward from a core. The mass is then blended with an immersion blender, blade kept below the surface so no air enters.
A split ganache can usually be rescued by adding warm liquid and blending again.