Preheat oven to 350F. Line the inside of an 8-in (23 cm) square pan with foil so that it goes up the sides to the rim. Lightly grease the foil with non-stick cooking spray.
Melt the butter, chocolate, and salt in a microwave safe bowl. Microwave for 2-3 minutes, stirring every 30 seconds.
Stir in sugar, then the eggs, one at a time.
Sift together the cocoa powder and corn starch in a small bowl then stir them into the chocolate mixture. Stir in the orange zest and cinnamon
Beat the batter vigorously for at least one minute, until the batter is no longer grainy but rather nearly smooth. Or beat it using a hand or stand mixer.
Add the nuts, then scrape the batter into the prepared pan.
Bake for 30 minutes, or until the brownies are set in the center. Remove from oven and let cool completely before removing from the pan and slicing.
Add grated zest on top, if desired.