Break canes into pieces. Place in a plastic bag. Squeeze bag to expel air. Wrap bag in a tea towel. Place on a hard surface. Pound with a rolling pin until canes are coarsely crushed.
Line a 25cm x 30cm swiss roll pan with baking paper. Spread dark chocolate over prepared tray. Dollop white chocolate in large circles over dark chocolate. Using a skewer or small knife, swirl white chocolate into dark chocolate.
Sprinkle crushed candy canes, salt and cocoa nibs (if using) over chocolate. Refrigerate for 20 minutes. Break into pieces. Serve.
NOTE: Store bark in an airtight container in a cool place (or the refrigerator) for up to 5 days.