2/3 cup heavy cream
1/2 cup light corn syrup
1/3 cup packed dark brown sugar
1/4 cup unsweetened Dutch-process cocoa powder
1/4 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 1/2-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.
Sauce can be made 1 week ahead and cooled completely, then chilled in an airtight container or jar. Reheat before using.