Heat oven to 350 degrees F. Line a 9-inch square baking pan with nonstick foil; set aside.
Reserve 2 ounces of the chocolate. Chop remaining 6 ounces and place in a large microwave-safe bowl with butter. Microwave for 1 minute; stir. Microwave 30 seconds, then stir until smooth.
Stir sugar, flour, baking soda and salt into chocolate mixture until well blended. Add eggs and vanilla; stir until smooth. Chop remaining 2 ounces chocolate. Stir into batter. Pour into prepared pan, spreading level.
Bake at 350 degrees F for 45 minutes or until toothpick comes out clean when inserted in center. Cool on rack for 10 minutes. Lift brownie from pan using foil; place directly on rack to cool completely. Dust with confectioners' sugar.
