Microwave chocolate and milk in large microwavable bowl on High 2-3 minutes or until chocolate is almost melted, stirring halfway through heating time. Stir until chocolate is completely melted.

Stir in vanilla. Spread in foil-lined 8" square pan. Drop peanut butter by teaspoonfuls on top of fudge. Swirl with knife to marbleize.

Refrigerate 2 hours or until firm. Cut into squares.