Best Cocoa Brownies

Name: Best Cocoa Brownies
Cook time: 45 min
Total time: 45 min
Cuisine: American
Ingredients: 10 tablespoons (140 grams) unsalted butter; 1 1/4 cups (250 grams) granulated sugar; 3/4 cup plus 2 tablespoons (65 grams, may vary by brands) unsweetened cocoa powder (natural or Dutch-process); 1/4 teaspoon salt (or a heaping 1/4 teaspoon flaky salt, as I used); 1/2 teaspoon pure vanilla extract; 2 large eggs, cold; 1/2 cup (65 grams) all-purpose flour; 2/3 cup (75 grams) walnut or pecan pieces (optional)