In a large heavy saucepan stir together the brown sugar and evaporated milk. Bring to a boil over medium-high heat. Cook until the temperature reaches between 112 to 116 degrees C with a sugar thermometer or until a small amount of syrup dropped into cold water forms a soft ball that flattens when removed from the water and placed on a flat surface.
Remove from the heat and stir in the butter, vanilla and walnuts.
Continue to beat with a sturdy spoon until the mixture loses its gloss. Pour into the prepared pan. Score into squares with a knife then set aside until firm; about 5 minutes.
Cut into squares and serve or store in an airtight container in the refrigerator.