Preheat the oven to 160 C (320 F). Grease and line an 8 inch square baking tin with baking or parchment paper, ensuring two sides overhang. In a small saucepan, place the butter and sugar, and place over medium heat, whisking occasionally, until melted and simmering. Add the vanilla extract and stir.
In a large bowl, place the eggs and vanilla, and whisk until well-beaten and combined. Add the melted and cooled butter and sugar to the bowl slowly, whisking constantly. Add the cocoa powder and whisk until the cacao powder has been absorbed. Using a handheld mixer, beat until very well-combined. Add the flour, cornstarch, baking soda, and salt, and mix with a spatula or spoon until just combined. Add the chopped chocolate or chocolate chips and the chopped walnuts. Mix until just evenly distributed throughout the batter. The batter will be thick and sticky.
Pour the brownie batter in the prepared tin and place in the oven. Bake brownies for approximately 30-35 minutes or until they no longer wet in the center.
Remove the brownies from the oven and allow them to cool in the pan at room temperature until the pan is no longer hot to the touch. For clean lines when you slice the brownies, place the brownies, still in the pan, in the refrigerator to chill for about an hour before removing them from the pan. These brownies are freezer friendly.