Prepare your baking sheet by lining it with parchment paper and lightly spraying with nonstick cooking spray.
In separate microwave-safe bowls, melt the white chocolate chips and dark chocolate chips in the microwave in 30-second intervals until smooth, stirring between each interval.
Pour the melted white chocolate onto the prepared baking sheet, spreading it evenly.
Drizzle melted dark chocolate over the white layer, then swirl together using a toothpick or skewer.
While still warm, sprinkle broken pretzel pieces and rainbow sprinkles generously on top.
Allow the bark to cool at room temperature for about an hour or refrigerate for 30 minutes until set.
Once firm, break into irregular pieces and serve.
