Whisk together the flour, cocoa, salt, and baking powder in a medium bowl. Set aside.
Combine the butter and unsweetened chocolate in a large microwave-safe bowl. Microwave on high in 30-second intervals, stirring in between, until melted. Let cool slightly.
Whisk in the sugars, eggs, Cold Brew Concentrate, and vanilla until smooth. Add the flour mixture and mix until just combined. Fold in the bittersweet chocolate. Spread the batter in the prepared pan and bake until a toothpick inserted in the center comes out with a few moist crumbs attached, 35 to 40 minutes.
Let cool and enjoy! (Try them warm with a scoop of vanilla ice cream!)