Line an 8 inch square baking pan with plastic wrap. In a saucepan, combine corn syrup and evaporated milk, add the chocolate. Heat on medium low until chocolate is melted. Remove from heat, stir in vanilla and icing sugar until smooth. Spread in pan. Drop peanut butter by teaspoonfuls onto fudge. With knife, swirl peanut butter through fudge to create a marble pattern. Refrigerate two hours or until firm.