Preheat your oven to 350 degrees F.
In a medium saucepan, combine the bittersweet chocolate, dark chocolate and butter over low heat. Gently stir the mixture until everything is melted together. Turn off the heat and add the cocoa powder and the sugar. Stir to combine.
Add the eggs, one at a time, and stir to combine until the eggs are fully incorporated. Add the vanilla and salt and combine. Add the flour and lightly stir until just incorporated.
Transfer the batter to a foil lined and sprayed 9x9 baking dish. Bake for 30-33 minutes in your pre-heated oven.
Remove from the oven and set aside to cool for at least 1 hour before cutting.
Dust with powdered sugar if desired before serving