1. Line the inside of an 8-inch (23 cm) square pan with foil so that it goes up the sides to the rim. Lightly grease the foil with butter or non-stick cooking spray. Preheat the oven to 350º (180ºC).
2. Melt the butter and chocolate (and salt, if using) in a medium saucepan over very low heat on the stove top, stirring constantly until smooth.
3. Remove from heat and stir in the sugar, then the eggs, one at a time.
4. Sift together the cocoa powder and corn starch in a small bowl then stir them into the chocolate mixture. Beat the batter vigorously for at least one minute, until the batter is no longer grainy and nearly smooth. It will pull away from the sides of the pan a bit.
5. Add the nuts, if using, then scrape the batter into the prepared pan.
6. Bake for thirty minutes, or until the brownies feel just set in the center. Do not overbake. Remove from oven and let cool completely before removing from the pan and slicing.