Line a baking tray with baking paper.
Melt down dark and white chocolate in seperate bowls. If using peppermint essence, mix it into the dark chocolate once melted.
Tip the melted dark chocolate onto the baking tray and smooth out with a spatula.
Dot the melted white chocolate onto the dark, then using a toothpick or chopstick swirl it around.
Finely chop pistachios and cranberries. Sprinkle overtop.
Put bark in fridge for a few hours to firm up, then break into shards.