Adjust oven rack to middle position. Preheat oven to 325°F.
Cover a 13x9-inch baking pan with 2 folded sheets of aluminum foil – the first sheet will go across one way and the second sheet should go the length of the over way and be perpendicular to the first sheet. Cut the foil long enough to hang over all edges by 3 inches. The foil will be used to lift the cooked brownies from the pan. Spray the foil-lined pan with nonstick cooking spray.
In a medium bowl, combine the flour, salt(s), and baking powder. Whisk well.
1¼ cups cake flour, ½ teaspoon kosher salt, ⅛ teaspoon coarse salt, ¾ teaspoon baking powder
Melt the butter in a large heatproof bowl. Add the chopped chocolate and stir until completely smooth and incorporated. If necessary, gently heat the chocolate/butter mixture, on HIGH, in 10-second increments, until the chocolate is melted. Stir often.
12 tablespoons unsalted butter, 6 ounces unsweetened chocolate
When the chocolate mixture is smooth, gradually whisk in the sugar. Add the eggs one at a time until thoroughly combined. Add the vanilla and whisk again.
2¼ cups granulated sugar, 4 large eggs, 1 tablespoon pure vanilla extract
Add the flour mixture in 3 additions. Fold gently with a rubber spatula after each addition until batter is smooth and fully incorporated.
Transfer the batter to the prepared pan, spreading it into the corners and smoothing the surface. If using nuts, sprinkle them evenly over the batter.
1 cup chopped toasted pecans or walnuts
Bake 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.
Transfer brownies to a cooling rack and cool to room temperature. Remove cooled brownies from the pan by lifting the foil overhang. Cut into 2-inch squares.