In a large bowl, combine cocoa and soda; blend in 1/3 c butter. Add boiling water; stir until well blended. Stir in sugar, eggs and remaining butter. Add flour, vanilla and salt. Stir in chocolate chunks and nuts, if desired. Pour into greased 13x9x2 pan. Bake at 350 degrees for 35-40 minutes.
Cool before cutting into squares. A pizza cutter works great for cutting these..