1. Preheat oven to 350°F (180°C). Grease a 9 x 13 x 2-inch (3.5 L) baking pan or two 8-inch (2 L) square baking pans.
2. Stir together cocoa and baking soda in a large bowl; stir in 1/3 cup (75 mL) butter. Add boiling water; stir until mixture thickens. Add sugar, eggs and remaining butter; stir until smooth. Stir in flour, vanilla and salt until well combined. Stir in chocolate chips. Pour into prepared pan.
3. Bake for 35 to 40 minutes for single pan or 30 to 35 minutes for two square pans or until brownies begin to pull away from sides of pan. Cool completely in pan on wire rack.
4. One-Bowl Buttercream Frosting - Beat butter in medium bowl. Add icing sugar and cocoa alternately with milk, beating to spreading consistency (additional milk may be needed). Stir in vanilla. Frost brownies and cut into squares. Makes about 36 brownies (and about 2 cups/500 mL frosting).