Creamy Brown-Sugar Fudge

Lightly oil an 8-inch square glass baking dish. Coarsely chop chocolate and butter and chill. In a 4-quart heavy saucepan cook sugars, cream, and salt over low heat, stirring constantly with a wooden spoon and washing down any sugar crystals clinging to side of pan with a brush dipped in cold water, until sugar is completely dissolved, about 10 minutes. Increase heat to moderate and boil mixture, without stirring, until a candy or digital thermometer registers 238 °F. Remove pan from heat (leave thermometer in pan) and add chocolate, butter, and vanilla (do not stir). Cool fudge, without stirring, in pan until thermometer registers 130 °F., about 30 minutes. With a hand-held electric mixer on high speed beat fudge until it turns pale, loses its gloss, and just begins to set, about 1 to 2 minutes. Immediately pour fudge into baking dish and smooth top with an offset spatula. Cool fudge completely. Chill fudge, covered, until firm, at least 6 hours, and up to 1 week. Cut fudge in baking dish into 3/4-inch squares. Fudge keeps, layered between sheets of wax paper in an airtight container and chilled, 1 month. Serve fudge chilled.