Preheat the oven to 350° and butter an 8-inch square baking pan. In a medium saucepan, combine the butter and unsweetened chocolate and cook over low heat, stirring frequently, until melted. Transfer to a medium bowl and let cool for 5 minutes. Using a whisk, beat in the sugar, vanilla and salt. Add the eggs and whisk until smooth. Add the flour and cocoa and whisk until smooth.
Scrape the batter into the prepared pan and bake for about 22 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached. Transfer the brownies to a wire rack and let cool completely.
In a small saucepan, combine the bittersweet chocolate, butter, corn syrup and salt and cook over low heat, stirring frequently, just until the chocolate is melted. Pour the warm glaze over the brownies and let cool at room temperature for 30 minutes, until set. Refrigerate until firm, about 30 minutes.
Run a knife around the edge of the pan and carefully transfer the entire brownie to a cutting board. Cut the brownie into 16 squares and serve.
The brownies can be stored in an airtight container at room temperature for up to 1 week.