Frosted Chocolate Brownies

Rich luscious chocolate  in every bite! 3/4 cup baking cocoa 1/2 teaspoon baking soda 2/3 cup butter or margarine, melted, divided 1/2 cup boiling water 2 cups granulated sugar 2 large eggs 1-1/3 cups all-purpose flour 1-1/4 teaspoons vanilla extract 1/4 teaspoon salt...