Line a 13x9-in. pan with foil; coat with cooking spray., In a heavy Dutch oven, combine sugar, milk and butter. Bring to a rapid boil over medium heat, stirring constantly. Cook and stir 5 minutes. Remove from heat., Stir in chocolate chips, marshmallows and chopped chocolate until melted. Fold in walnuts and vanilla. Immediately spread into prepared pan. Drizzle with melted white baking chocolate; cool completely., Using foil, lift fudge out of pan. Remove foil; cut fudge into 96 squares. Store between layers of waxed paper in airtight containers.