Break the chocolate into pieces and place it in a medium sized bowl, cut the butter into cubes and put it in a separate bowl.
Place the whipping cream, muscovado sugar, blackcurrant puree and stout in a saucepan and heat until it comes to a boil and the sugar has dissolved. Remove the pan from the heat.
Pour the hot liquid over the chocolate and stir gently until all of the chocolate has melted, then add the butter and stir gently until all melted.
The ganache will seem quite runny, but it will set to a silky smooth ganache that is perfect for spreading.