Preheat the oven to 350 F.
Spray 9" x 13" baking dish with cooking spray. Line with parchment paper, leaving some overhang on the ends of the dish.
In a large microwave-safe bowl, add the butter and 10 oz. of the chocolate. Microwave on HIGH in 1-minute intervals until melted.
Meanwhile, in a separate medium-sized bowl, mix together the flour, cocoa powder, and salt. Set aside.
Stir in the sugar and brown sugar into the melted chocolate/butter mixture. Stir until sugars have dissolved.
Add the eggs, one at a time, whisking until fully incorporated. Stir in the vanilla.
Stir in the flour/cocoa mixture, stirring continuously until completely blended.
Chop the remaining 2 oz. of chocolate with a large knife, or food processor. Stir into the batter.
Pour the batter into the prepared dish and bake for 30 minutes, or until an inserted toothpick comes out dry (a little wetness is okay). Transfer to a wire rack and let cool. Sprinkle coarse sea salt over the top, if using.
Use your hands to carefully use the overhanging parchment paper to lift the brownies of the dish onto a cutting board.
Cut the brownies into 12 bars.