While there are plenty of choices when making brownies that will affect the final taste and texture, the most important is the choice between chocolate and cocoa powder.
Unlike cocoa, chocolate contains both cocoa solids and butter and plenty of fat by itself, which results in a less concentrated flavor. This produces very different brownies.
The flavor of chocolate brownies will also be noticeably less chocolate-forward than with cocoa. Being cut by the butter, its taste is less intense than concentrated cocoa powder.
Not only is cocoa powder more concentrated than chocolate, but since it has less moisture, you need fewer additives to balance it out, compounding the cocoa flavor even more.