When making ganache the first thing you want to do is chop up your chocolate into small pieces, this will help the chocolate melt. Place your cutup chocolate into a large mixing bowl.
Put your double cream into a saucepan and bring it to the boil.
As soon as it's boiling, pour it over the top of your chocolate. Let it sit for a few seconds and start to stir it round.
You want to continue to stir until all of your chocolate has melted and you have a smooth glossy mixture. You can also use a hand blender (the type you use for making smoothies), this really speeds up this process.
If at anytime your ganache starts to separate or look extremely greasy, take a small amount of milk and heat it up in the microwave or on the hob. Slowly add the hot milk into your mixture and mix it through.
Once all of your chocolate has melted you want to leave your ganache to firm up to a spreadable consitanvy. Placing a pieces of cling film over the top of your ganache, will stop the top of your ganache setting hard. Push the cling film down until there are no air gaps.
After 1-2 hours at room temperature your ganache will be a spreadable consistency, a bit like the consistency of Nutella. You can also use your electric hand whisk at this point if you would like it a little softer and more fluffy. This will beat air into your ganache and give you a similar look to buttercream.