Preheat the oven to 350ºF. Line a 9-inch square metal cake pan with foil, draping the foil over the edges. Lightly butter the foil.

In a large saucepan, melt the butter with the unsweetened chocolate over very low heat, stirring occasionally. Remove from the heat. Whisking them in one at a time until thoroughly incorporated, add the cocoa, sugar, eggs, vanilla and flour. Pour the batter into the prepared pan and smooth the surface. Sprinkle the salt evenly over the batter. Using a butter knife, swirl the salt into the batter.

Bake the brownies in the center of the oven for about 35 minutes, until the edge is set but the center is still a bit soft and a toothpick inserted into the center comes out coated with a little of the batter. Let the brownies cool at room temperature in the pan for 1 hour, then refrigerate just until they are firm, about 1 hour. Lift the brownies from the pan and peel off the foil. Cut the brownies into 16 squares. Serve at room temperature.

Note: These can be refrigerated for up to 3 days, and frozen for up to 1 month. Good luck with keeping them around that long.