Place the two varieties of chocolate in a bowl that is big enough to fit the puree as well.
Place the morello cherry puree in a saucepan with the sugar and bring to a boil. Turn the heat off and give the puree about 30 seconds to cool down.
Pour the puree over the chocolate and stir until combined and the chocolate is melted. Add the butter and stir until melted and incorporated.
Place a layer of clingfilm over the ganache, make sure it touches the surface of the ganache and leave it to cool. Do not place the ganache in the fridge as it will set too hard.