Vegetable oil for greasing
2 cups packed light brown sugar (14 oz)
1 cup heavy cream
1/2 stick (1/4 cup) unsalted butter
1/4 teaspoon salt
1/4 cup plus 1 teaspoon dark rum
1/4 teaspoon vanilla

Special equipment: parchment paper; a candy or deep-fat thermometer

Line bottom and sides of an 8-inch square baking pan with parchment paper and oil parchment.

Bring brown sugar, cream, butter, salt, and 1/4 cup rum to a boil in a 3- to 4-quart heavy saucepan, stirring until butter is melted, then boil over moderate heat, stirring frequently, until thermometer registers 248F (firm-ball stage), about 15 minutes. Remove from heat and stir in vanilla and remaining teaspoon rum. Pour into baking pan and cool completely until firm, 1 to 2 hours. Invert caramel onto a cutting board, then discard parchment and turn caramel glossy side up. Cut into 1-inch squares.

Cooks' note:
Caramels keep, layered between parchment, in an airtight container 1 week.