Butter an 8- or 9-inch square baking dish.

Stir together brown sugar and evaporated milk in a heavy saucepan. Bring to a boil over medium-high heat. Cook until the temperature reaches between 234-240°F (112-116°C) or until the syrup forms a soft ball in cold water.

Remove from heat and stir in walnuts, butter, and vanilla. Beat until the mixture loses its gloss.

Pour into the prepared pan. Score into squares and let it set until firm for about 5 minutes.