A little digging found that, depending on the brand and fat content, 1 cup of cocoa powder can weigh anywhere from 80 to 118 grams. I used Valrhona, which is on the heavy end; for comparison, I weighed 1 cup of ubiquitous Fry's, which came to 92 grams. Moral of the story: Use a scale.
Adapted from Dorie's Cookies by Dorie Greenspan.
Servings: 24 2-inch brownies
Ready time: 45 minutes
1 cup plus 2 tbsp (150 g) all-purpose flour
1 cup (112 g) best-quality, Dutch-processed cocoa powder, plus more to dust pan
11 tbsp (150 g) unsalted butter, softened, plus more to grease pan
2 1/2 cups (500 g) granulated sugar
1/2 tsp fine sea salt
4 large eggs, lightly beaten, at room temperature
1 tsp vanilla extract
Preheat oven to 325 F. Butter the sides and smear an X on the bottom of a 9-by-13-inch baking pan. Dust sides with cocoa powder, tapping out excess. Line bottom with parchment paper.
Sift together flour and cocoa powder into a medium bowl. Set aside. In stand mixer fitted with paddle, beat butter, sugar and salt on medium-low speed until smooth, stopping to scrape down bowl, about 3 minutes. Stop mixer and change to whisk attachment. With mixer on medium-high, slowly pour in eggs and vanilla. Beat for 5 minutes, stopping to scrape down bowl once. Using silicone spatula, gently fold in dry mixture by hand until batter is uniformly wet. Pour into prepared pan and smooth out top.
Bake on middle rack, rotating pan at halfway point, until toothpick inserted in centre comes out clean, 35 to 40 minutes.
Remove from oven. Cool completely on wire rack. Loosen sides with knife. Cut brownies into 2-inch squares.