Preheat the oven to 350 degrees F. Prepare an 8 by 8-inch square baking dish with nonstick cooking spray.
In a double boiler, or a microwave set on low power, melt the butter and chocolate, stirring frequently. Transfer the chocolate mixture to a large bowl and let cool. Whisk in the cocoa until smooth. Whisk in the granulated sugar. Whisk in the eggs and vanilla. Fold in the flour and salt until just blended.
Pour the batter into the prepared pan. Bake until the brownies are slightly overdone (they should be gooey inside), 20 to 25 minutes. Let cool in the pan on a wire rack for 5 minutes.