Preheat oven to 350 degrees. Line an 8 by 8-inch pan with parchment paper and spray with cooking spray.
Combine flour, cocoa powder, salt, cornstarch, and baking powder in a medium bowl.
1/2 cup All Purpose Flour, 1/3 cup Unsweetened Cocoa Powder, 1/2 teaspoon Salt, 1/4 teaspoon Baking Powder, 1 Tablespoon Cornstarch
In a medium saucepan, melt the butter. Remove from the heat and whisk in sugar, eggs and vanilla.
1/2 cup Melted Butter, 1 cup Granulated Sugar, 2 Eggs, 1 teaspoon Vanilla
With a rubber spatula, stir in the dry ingredients. Mix in chopped walnuts. Spread the brownie mixture evenly into the prepared baking dish. Bake for 20 - 25 minutes being careful not to over bake.
1/2 cup Chopped Walnuts