Preheat oven to 350 degrees F. Coat a 13 x 9-inch pan with nonstick spray.

Whisk flour, cocoa and salt together into a small bowl to aerate and combine; set aside.

In a large bowl with an electric mixer on medium-high speed, beat butter until creamy, about 2 minutes. Add sugar and brown sugar gradually, beating until light and fluffy, about 3 minutes, scraping down the bowl once or twice. Beat in vanilla, then beat in eggs one at a time, allowing each one to be absorbed before adding the next. Add about one-third of flour mixture and mix on low speed. Gradually add remaining flour mixture, mixing just until blended. Batter will be thick. Chop one 6-ounce package of blackberries finely and fold into batter, along with chopped chocolate. Spread evenly into prepared pan.

Bake for about 30 minutes or until bars are slightly puffed and the edges have begun to come away from the side's of the pan. A toothpick inserted in the center comes out with some moist crumbs clinging. Place pan on rack until cool.

Place chopped chocolate in a heatproof bowl. Bring cream to a boil in a saucepan over medium heat. Immediately pour over chocolate; allow to sit for a few minutes for heat of cream to melt chocolate, then stir gently until ganache is smooth. Gently whisk in softened butter until ganache is smooth; whisk in brandy, if using. Allow to cool at room temperature until thick enough to spread, about the consistency of soft mayonnaise.

Spread ganache evenly over brownies using a small offset spatula. Refrigerate until serving time, up to 2 days; bring to room temperature before serving. Cut into 32 bars (8x4). Place a single blackberry or raspberry on top of each brownie bite before serving.