1. Heat oven to 350°F. Grease 13x9x2-inch baking pan.
2. Stir together cocoa and baking soda in large bowl; stir in 1/3 cup butter. Add boiling water; stir until mixture thickens. Stir in granulated sugar, eggs, remaining 1/3 cup butter and vanilla; stir until smooth. Add flour and salt; blend completely. Stir in chocolate pieces. Spread in prepared pan.
3. Bake 35 to 40 minutes or until brownies begin to pull away from sides of pan. Cool completely in pan on wire rack. Dust with powdered sugar, if desired. Cut into bars. Makes 24 to 36 brownies.