Preheat the oven to 350°F. Lightly grease a 9×13 baking dish and line with parchment paper. Set aside.
Sift the flour, cocoa powder and salt and place in a mixing bowl. Set aside.
Pour the hot, melted butter oil and sugar in another medium bowl and whisk really well.
Add the eggs and vanilla and whisk an additional minute. Add the cocoa mixture and slowly whisk together until just combined.
Pour into the prepared baking dish and top with chopped chocolate.
Bake for 28-32 minutes or until no the middle no longer jiggle (these will be fudgy so a toothpick check in the middle will still be dirty – just give it a shake to make sure it's not jiggling and it should be done).