Coat a 5-by-10-inch loaf pan with cooking spray. Line with plastic wrap leaving a 2-inch overhang on 2 sides.
Bring evaporated milk, brown sugar, butter, and salt to a boil in a medium saucepan, stirring constantly. Reduce heat to medium-low, and simmer, stirring frequently, until mixture registers 236 degrees on a candy thermometer, about 25 minutes.
Transfer to a mixer bowl, and beat in confectioners' sugar on low speed. Scrape down sides of bowl as needed. Increase speed to medium, and beat until mixture is thickened and smooth, 2 to 4 minutes. Reduce speed to low, and add vanilla and walnuts.
Spread mixture in pan, smoothing top. Refrigerate, uncovered, until firm, about 25 minutes. Unmold fudge using plastic overhang, and discard plastic. Cut into 18 pieces.
Penuche will keep, covered, for up to 1 week.