Line an 8 x 12" baking pan with aluminum foil, leaving foil hanging over the sides. Spray generously with non-stick cooking spray.
Preheat oven to 350°.
In a large bowl, melt the butter and 2 oz. of the baking chocolate. Microwave in 15-30 second intervals, stirring occasionally until melted.
Whisk in the granulated sugar and the brown sugar.
Whisk in 4 eggs and vanilla until smooth.
In a medium bowl, sift together the flour, cocoa powder, and salt.
Slowly whisk the flour mixture into the wet ingredients. The batter will be very thick.
Coarsely chop the remaining baking chocolate and gently fold it into the brownie batter.
Spread the brownie batter evenly into the prepared pan. Bake for 25-30 minutes, until a toothpick inserted in the middle comes out clean.
Allow the brownies to cool for 15 minutes before removing from the pan. The longer you let them cool the more set they will become.