Preheat oven to 325°F. Line a 8x8x2 metal pan with parchment paper.
Over medium heat, melt 10 tablespoons unsalted butter. Stir frequently as it continues to bubble. When the amount of bubbles subside, there are small brown specs, and it smells like nutty caramel, remove the pan from heat. Do not let the butter burn.
Add 1¼ cups granulated sugar, ¾ cup natural unsweetened cocoa powder, and ½ teaspoon kosher salt to the browned butter and stir well to combine.
After the mixture has cooled in the pot and is only slightly warm to the touch, add the 2 large eggs and 2 teaspoons vanilla extract, then stir well to combine.
Add 1/2 cup all purpose flour and ¼ teaspoon baking powder, stir to combine. Finally, stir in the 1 cup walnut halves (you can reduce the amount of nuts or omit entirely if you want).
Transfer batter to the parchment lined baking dish. It will be thick and may not spread all the way to cover the base of the baking dish. You can use a spatula to flatten it out if you want, but the batter will spread slightly as it bakes. Bake in the preheated oven for 25 minutes.
When done baking, lift the parchment paper with the brownies out of the pan and set them on the counter or a rack to cool. Once cooled, cut into pieces with a sharp knife.
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Notes
1. Measure cocoa powder as you would flour - by scooping and leveling.2. Feel free to reduce the amount of walnuts. The amount listed in the recipe is a lot.