Line an 8 inch square pan with foil. Butter foil and set aside. Combine peanut butter chips and 2/3 cup milk in a saucepan. Heat over medium heat, stirring, until smooth. Stir in ½ tsp vanilla. Pour into prepared pan. Refrigerate for 10 minutes. Meanwhile, combine chocolate chips and remaining milk in saucepan and heat, stirring, until smooth. Stir in remaining vanilla. Spread over peanut butter layer. Refrigerate for 1 hour or until firm. Using foil, remove fudge from pan. Cut into 1 inch squares.