Butter a 8 or 9 inch square baking dish.

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 234 and 240 degrees F (112 to 116 degrees C), 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 looses 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.