Line a 18cm x 18cm brownie tin or similar-sized square food box with cling film. Put the dark chocolate and butter in a heatproof bowl and microwave until melted, stopping to stir the mixture every 30 seconds. Add in the sour cherries along with the caramel. Mix everything together then put into your prepared tin. Chill in the fridge until set (around 1 hr) then turn it out onto a board and remove the cling film. Chop into squares to serve.