Line an A4-sized baking sheet with parchment. Break the dark or milk chocolate into a large glass bowl. Bring a small saucepan of water to a slow simmer, then sit the bowl on top, making sure it doesn’t touch the water. Over a low heat, slowly melt the chocolate, stirring once it begins to melt, or melt in the microwave in short bursts until smooth and glossy.
Pour the chocolate onto the prepared baking sheet, spread into an even layer using a palette knife, then melt the white chocolate following the same method. Dot blobs of the white chocolate on top of the dark chocolate then use a skewer to swirl them together in any pattern you like.
Top the bark with any assortment of treats you fancy – the more outrageous the better. Leave to set at room temperature to get a glossy effect, then break into shards to serve. Will keep for two weeks in an airtight container.