Butter a 15x10x1-inch baking pan. Set prepared baking pan aside.
In a heavy 3-quart saucepan, combine brown sugar, corn syrup, and the 1/2 cup whipping cream; mix well. Cook over medium-high heat to boiling, stirring constantly to dissolve the sugar. Carefully clip candy thermometer to side of pan.
Cook over medium heat, stirring frequently, until candy thermometer registers 250 degree F (hard-ball stage). Mixture should boil at a moderate, steady rate over the entire surface. Remove from heat. Remove the thermometer from pan.
Add the butter, the 2 tablespoons whipping cream, and the vanilla; mix well. Pour the mixture into the prepared baking pan. Cool.
Cut the candy into about 1-1/2x1/2-inch rectangles. Roll each piece of candy in waxed paper or plastic wrap. Store candy, tightly covered, in a cool, dry place. Makes 2-1/4 pounds candy.