Preheat oven to 350° F
Line an 8-inch baking pan with tin foil. Spray tin foil with nonstick cooking spray.
Melt bittersweet and unsweetened chocolate along with butter in a bowl over a pan of hot but not boiling water. Stir until melted.
Remove from heat and whisk in cocoa powder.
Let cool slightly.
Combine eggs, sugar, vanilla and salt in a bowl and whisk until blended.
Whisk in the warm chocolate mixture.
Stir in the flour with a wooden spoon until combined.
Pour evenly into the prepared tin foil lined pan.
Bake for 30-40 minutes or until a toothpick comes out clean when inserted in the center.
Lift from pan using the tin foil as a handle.
Cut into squares and enjoy!
May be chilled and wrapped in plastic for up to five days.