LINE 8-inch-square baking pan with foil.
COMBINE sugar, evaporated milk, butter and salt in medium, heavy-duty saucepan. Bring to a full rolling boil over medium heat, stirring constantly. Boil, stirring constantly, for 3 minutes. Remove from heat and stir in vanilla extract and marshmallows. Stir vigorously for 1 minute or until marshmallows are melted.
WORKING FAST, POUR 1/2 cup hot mixture into glass measure. Add white morsels (do not stir); set aside.
WORKING FAST, STIR in semi-sweet morsels into saucepan until morsels are just melted. Pour into prepared pan.
STIR white morsel mixture until morsels are just melted.
DRIZZLE white morsel mixture over semi-sweet chocolate mixture; swirl with knife for marbled effect. Refrigerate for 2 hours or until firm. Lift from pan; remove foil. Cut into 48 pieces.