Line an 8-inch square pan with foil; grease foil. Stir milk and butter in a medium saucepan over low heat until butter melts and mixture is smooth. Put 1/3 cup into a small bowl. Add chocolate chips and stir until melted and smooth. Add peanut butter chips to mixture in saucepan; stir until melted and smooth, returning saucepan to heat if needed.

Spread saucepan mixture in lined pan. Drop teaspoonfuls of chocolate mixture over the top. Using a small rubber spatula, lift some of the mixture from bottom of the pan over the chocolate mixture, creating a decorative pattern. Refrigerate 3 hours or until firm. Invert pan, peel off foil, invert fudge and cut in 1-inch squares.