Line an 8-in. square pan with foil and coat with cooking spray; set aside. In a small heavy saucepan over low heat, melt chocolate chips with milk; stir until smooth. Remove from the heat; cool for 2 minutes. Stir in vanilla. Fold in marshmallows and graham crackers. , Pour into prepared pan. Refrigerate for 1 hour or until firm., Using foil, lift fudge out of pan. Discard foil; cut into 48 pieces.