Preheat oven to 350°F (165°C). Grease 9x13-inch (25x40 cm) baking pan.

For the brownies, cream together granulated sugar, cocoa powder and butter until fluffy. Add beaten eggs and vanilla. Add flour and salt. Stir in nuts or chocolate chips, if using.

Spread brownie batter in greased baking pan and bake for 30 minutes, just until the top appears dry. Do not overbake. They may appear to be underdone but you want these to stay moist.

As soon as the brownies are done baking, mix the frosting ingredients together with an electric mixer until smooth. If frosting is too thick, mix in boiling water, one teaspoon at a time, until desired consistency is reached. Working quickly, immediately spread frosting over warm brownies. Cool brownies completely in pan on a wire rack. Cut into squares. Store tightly covered at room temperature for up to three days.