Ganache serves as a base for many desserts. However, it can go from smooth and glossy to split and oily in a flash, especially if you make the mistake of over-stirring it.
At its core, ganache is a mixture of ingredients that don't naturally bind, so a lot of stirring is required. Once it is at that point, however, stop stirring immediately.
Over-stirring can cause the chocolate’s oils and fats to separate, making it grainy. Stopping once it’s mixed smoothly lets it begin its natural setting process.
Ganache hardens as it cools; by ceasing stirring, it cools as a cohesive whole without separating. If you over-stirred your ganache, there are ways to fix it.
It might sound counter-intuitive, but you’ll need to continue stirring. The trick is to mix in additional liquid so the chocolate fats have something to emulsify.