Prepare your baking pan by cutting a strip of parchment paper the same width of the pan. Grease the pan with butter or baking spray and stick the parchment into the pan, leaving an overhang on each side.
Make the batter
In a medium bowl, whisk together the cocoa, flour and salt. Set aside.
In a microwave safe mixing bowl, melt the butter. You can also melt it on the stovetop and transfer to a mixing bowl.
Add the brown sugar to the butter and mix well.
Add the eggs and vanilla to the butter mixture and mix until incorporated.
Add the flour mixture to the butter mixture and mix until fully combined.
Bake
Put the batter into the baking pan and spread it into an even layer.
Bake for about 20 minutes, until the internal temperature of the brownies is 180°F (82°C). They will still be gooey on the inside and a toothpick will come out coated, but they will firm up as they rest.
Let the brownies cool fully in the pan before trying to take them out. You can try to cut and serve directly in the pan, or use the parchment sling to gently pull them straight up out of the pan and cut them on a cutting board.
Store
These will be best the day after baking if they are kept uncut in the baking tray. They will keep well uncut for several days.
Once they are cut, eat the same day, or wrap individually and freeze in an airtight container.