3/4cupplus 2 tablespoons all-purpose flour, divided into 3/4 cup and 2 tablespoon portions
1 1/2cupsfresh blueberries, divided into 1 cup and 1/2 cup portions
Preheat the oven to 400 (F). Prepare an 8 by 8-inch baking pan by lining with parchment paper or foil, leaving an overhang over each end.
In a double boiler (or, a heatproof bowl set over a pot of gently simmering water), combine 5 ounces finely chopped bittersweet chocolate and 6 tablespoons unsalted butter. Melt over medium-low heat, using a heatproof rubber spatula to stir occasionally until the mixture is melted and smooth. Once the chocolate and butter have completely melted, remove from heat and allow to cool slightly on a wire rack.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, combine 2 large eggs and 1 cup granulated sugar. Beat on medium-high speed until light and fluffy, about 5 minutes. Lower the speed to the mixer's lowest setting and add 1 teaspoon pure vanilla extract. Continue mixing on the lowest speed and slowly add the cooled melted butter and chocolate mixture (from the 2nd step) into the mixture, mixing until just combined and the batter is a uniform chocolate color.
Once the batter is a uniform chocolate color, sprinkle 3/4 cup all-purpose flour and 1/2 teaspoon kosher salt over the surface of the batter. Use a rubber spatula to gently fold the flour into the chocolate batter — DO NOT OVERMIX. Overmixing will lead to dense, hard brownies and me crying tears of sadness. Simply fold until the ingredients are just combined — at this point, it's okay to have one or two flour streaks left in the mixture.
In a separate medium bowl, toss together 1 cup fresh blueberries with the remaining 2 tablespoons all-purpose flour. Add the blueberry mixture into the brownie batter, using a rubber spatula to fold the fruit evenly into the batter. Again, be careful not to overmix!
Transfer the batter into the prepared baking pan and bake in the preheated oven for 30 to 40 minutes, until the brownies are slightly puffed but still moist in the center. Transfer the pan to a wire rack and immediately press the remaining 1/2 cup of blueberries on to the top of the brownies. Be careful not to press too hard — a gentle tap will do. Allow the brownies to cool in their pan completely before cutting into squares.