Cut dark chocolate into smaller pieces and melt dark chocolate pieces and unsalted butter in microwave on 50% power in 20 seconds increment until chocolate is completely melted. Stir every 20 seconds.
Bring melted chocolate and unsalted butter to a medium bowl, then add granulated sugar, brown sugar, vanilla extract and eggs. Mix well to combine.
Add salt, all purpose flour and cocoa powder to bowl. Mix well to combine.
Pour brownie batter into a 9"x9" square dish lined with parchment paper. Highly recommend using parchment paper because it will make it easier to lift the brownie out of the dish later.
Bring brownie to oven and bake for 40 minutes. At the 40 minute mark, remove and slide a toothpick into the center of the brownie to check for doenness. If toothpick comes out clean, brownie is done. If it does not come out clean, bring brownie back to the oven and bake for another minute to two until toothpick comes out clean.
Remove brownie from oven, and let it cool for 5 minutes. Lift parchement apper and brownie out of the dish and let brownie cool on a cooling rack for about an hour. You can speed up the cooling process by bringing brownie to the fridge (but do not cover because condensation will happen).