Melt the dark and white chocolate in separate bowls suspended over pans of gently simmering water, stirring every now and then. Meanwhile, line a large baking tray with baking parchment. Leave the chocolate to cool for 10 mins.
Pour the dark chocolate onto the tray and spread it into a rectangle, roughly 20 x 25cm. Drizzle over the white chocolate and create a swirled pattern by dragging a cocktail stick back and forth across the chocolate. Scatter over the nuts, fruit and sea salt, then leave somewhere cool to set.
Break into shards or leave whole, wrap in cellophane or pack into a box, then tie up with ribbon. Will keep for 2 weeks.