Line an 8x8x2" or 9x9x2" baking pan with foil, extending foil over edges of pan. Butter or spray the foil. Set aside.
In a heavy saucepan with high sides melt butter over low heat. Add the brown sugar, sweetened condensed milk (or cream, if using) and corn syrup; mix well. Cook and stir over medium-high heat until mixture boils. Insert a candy thermometer. Reduce heat to medium; continue boiling at a moderate, steady rate, stirring frequently, until the thermometer registers 248°F, firm ball stage. Adjust heat as necessary to maintain a steady boil.
Remove pan from heat; remove thermometer. Stir in vanilla. Quickly pour mixture into prepared pan. When firm, use foil to lift out of pan. Cut into 1" squares. You can also press each piece lightly onto a small plate of fleur de sel or sea salt, wrap as desired. Makes 2 lbs, or about 64 pieces.
Let cool for about 3 hours before cutting. Cut with a sharp knife. I used a pizza cutter and it worked better than a knife.