Heat oven to 350 degrees. Grease one 13x9x2-inch pan or two 8x8x2-inch square pans. 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(s). Bake 35-40 minutes for rectangular pan, 30-35 for square pans or until brownies begin to pull away from sides of pan. Cool completely in pan(s); frost with One-Bowl Buttercream Frosting. Cut into squares.
In small mixer bowl cream butter. Add coca and powdered sugar alternately with milk; beat to spreading consistency (additional milk may be needed). Blend in vanilla.