Preheat oven to 350.

Prepare 13 x 8 pan with tin foil and then spray with cooking spray.

Set aside.

In a double boiler set over medium heat, melt the butter the bittersweet chocolate and the unsweetened chocolate.

Once melted, set aside to cool.

In a large bowl, whisk together the eggs, sugar, then add the salt and the vanilla extract.

Combine the flour and the cocoa in a small bowl.

Pour the melted chocolate mixture into the egg, sugar mixture and whisk until smooth and combined.

Add the flour and stir until just combined, do not over mix.

Pour the batter into prepared pan.

Cook for 23-30 minutes. Do not over bake.