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Adjust oven rack to middle position and preheat to 350°F. Line an 8x8 baking dish with foil and lightly coat with cooking spray.
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Place butter, bittersweet chocolate, and unsweetened chocolate in double boiler bowl. Gently heat until just melted, stirring occasionally. Remove from heat and let cool slightly.
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Using a hand mixer, whisk sugar, eggs, salt, and vanilla in large bowl until light. Whisk in chocolate mixture until thick and glossy. Add flour and cocoa and stir until just combined.
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Pour into prepared pan and bake until just set, about 30 minutes. Let cool completely in pan before gently lifting them out using the foil overhang.