Put the chocolate into a heatproof bowl and place over a pan of ‘just simmering’ water, but don’t let the bowl touch the water. Stir occasionally until melted.
Line a tray with baking paper and pour the chocolate on to it, then spread with a spatula or palette knife into a rectangle about 18x22cm and around 0.5cm thick.
Immediately sprinkle the seeds, pistachios and cranberries evenly over the top.
Transfer to the fridge to set for at least one hour. Once set, break the bark up into jagged pieces.