Butter 15 x 10-inch jelly roll pan. Line with parchment. Melt 1 lbs white chocolate with peanut butter in large bowl set over saucepan of simmering water, stirring occasionally until chocolate is melted and smooth.
Meanwhile, melt bittersweet chocolate in medium bowl set over saucepan of simmering water, stirring occasionally. Melt remaining 2 oz. white chocolate in small bowl set over saucepan of simmering water, stirring constantly.
Remove all bowls from over water. Pour peanut butter mixture onto prepared pan, spreading to cover surface completely. Using spoon, drizzle melted bittersweet chocolate in lines over peanut butter mixture.
Draw tip of small sharp knife through chocolate-drizzled mixture to marbleize. Using spoon, drizzle melted white chocolate over. Draw tip of knife through chocolates to marbleize. Chill until firm, at least 2 hours or over-nigh Tbsp Cut into 2-inch triangles or other shapes. Cover and chill. Serve cold.Servings: