Line cookie sheet with parchment paper. In medium saucepan over low heat, melt bittersweet chocolate just until soft. Stir until smooth. Cool 5 minutes.
Stir in nuts. Spread mixture on paper-lined cookie sheet to 1/4-inch thickness. Drizzle with melted white chocolate baking bar; swirl with toothpick. Refrigerate 30 minutes or until firm. Break bark into pieces.