Preheat oven to 350 degrees F. Line 8-inch square baking pan with foil, leaving some overhang over the edge of pan. Spray the foil with cooking spray.
Put butter in a microwave safe bowl, medium sized. Break up the unsweetened chocolate into several pieces and add to the bowl with the butter. Microwave the butter and chocolate for one minute on high power. Remove from the microwave and stir for about a minute or so until all the chocolate and butter is entirely melted and no lumps of chocolate remain.
Add sugar to the butter-chocolate mixture and stir until thoroughly combined. Add eggs and stir until thoroughly combined.
Stir in salt and vanilla. Add cocoa and stir to combine.
Add the flour and stir until flour is incorporated into the batter.
Pour batter into the prepared pan. Spread out the batter as needed to evenly cover the bottom of pan.
Bake in the oven for 25 to 30 minutes until brownies are set in the center. When done, there's no jiggle in the brownies and toothpick inserted in the center comes out clean or with fudgy residue. If the toothpick emerges with wet looking batter, I give it another couples minutes.
Once brownies are done, allow to cool completely in the pan for several hours. Once cooled, use foil sleeve to remove brownies from pan to cutting board.
Then remove foil: 1) Either hold slab perpendicular to the cutting board, peel off foil, then set brownies down. 2) Or flip brownies upside down on cutting board, peel off foil, and flip slab right side up. You can use a second cutting board to help with the flips if you prefer.
Slice brownies into desired size for serving.