Hot Fudge Sauce

⅔ cup heavy cream
½ cup light corn syrup
⅓ cup packed dark brown sugar
¼ cup unsweetened Dutch-process cocoa powder
¼ teaspoon salt
6 oz fine-quality bittersweet chocolate (not unsweetened), finely chopped
2 tablespoons unsalted butter
1 teaspoon vanilla

Bring cream, corn syrup, sugar, cocoa, salt, and half of chocolate to a boil in a 1 to 1 ½-quart heavy saucepan over moderate heat, stirring, until chocolate is melted. Reduce heat and cook at a low boil, stirring occasionally, 5 minutes, then remove from heat. Add butter, vanilla, and remaining chocolate and stir until smooth. Cool sauce to warm before serving.