Begin preheating the oven to 325° F (163° C) and either line an 8x8 metal baking pan with parchment and spray with cooking spray or use cooking spray with flour. Overhang the parchment about an inch, if using.
Add the butter to a large microwave-safe mixing bowl and melt in the microwave. With a spatula or spoon, stir in the cocoa powder and mix very well until it is entirely smooth and lump-free.
Add the sugar, salt, eggs, and vanilla and either whisk or stir very well. The mixture will be thick so use some elbow grease to get things smooth.
Add in the flour and cornstarch then fold and blend until just combined.
Pour the batter into the baking pan and smooth it out evenly. Sprinkle the walnuts atop if using.
Place the pan in the oven and bake for 30 minutes give or take, until a toothpick comes out with several moist crumbs. The edges will be set, the middle will be lightly undercooked.
Cool the brownies completely in the pan then serve. Store cut and loosely covered to maintain chewiness.
Notes
Adapted from Foodess and using suggestions from around the web.