Heat oven to 350. Grease rectangular pan, 13x9x2 inches. In medium bowl combine cocoa and baking soda. Blend in 1/3 cup melted butter. Add boiling water; stir until mixture thickens. Stir in sugar, eggs and remaining 1/3 cup melted butter; stir until smooth. Add flour, vanilla and salt; blend completely. Stir in chocolate chips. Pour into prepared pan and bake 35 to 40 min, or until brownies begin to pull away from sides of pan. Cool completely in pan; frost with one bowl buttercream frosting. Cut into squares.