In a mixing bowl, combine cocoa and baking soda; blend in one third cup melted butter. Add boiling water, stirring until thickened. Stir in sugar, eggs, vanilla and remaining butter. Add flour, cinnamon and salt. Fold in chocolate chips. Pour into a greased 33 cm x 23 cm x 5 cm baking pan. Bake at 180ºC or 40 minutes or until brownies test done. Cool. For frosting, cream butter in a mixing bowl. Combine cocoa, sugar and cinnamon; add alternately with the milk. Beat to a spreading consistency; add vanilla. Add more milk if necessary. Spread over the brownies.