The Best Classic Brownie

1 cup pecans or walnuts, chopped (optional)
1 1/4 cups cake flour
1/2 teaspoon salt
3/4 teaspoon baking powder
6 ounces unsweetened chocolate, chopped fine
12 tablespoons (1 1/2 stick) unsalted butter, cut into six 1 - inch pieces
2 1/4 cups sugar
4 large eggs
1 tablespoon vanilla extract

Adjust the oven rack to the middle position; heat oven 325 degrees. Spray pan with nonstick cooking spray.

If using nuts, spread nuts evenly on rimmed baking sheet and toast in oven until fragrant, 5 to 8 minutes. Set aside to cool.

Whisk together cake flour, salt and baking soda in medium bowl; set aside.

Melt chocolate and butter in a large heatproof bowl over saucepan of simmering water, stirring occasionally until smooth. When chocolate mixture is completely smooth, remove bowl from saucepan and gradually whisk in sugar. Add eggs one at a time, whisking after each addition until thoroughly combined. Whisk in vanilla. Add flour mixture in three additions, folding with rubber spatula until batter is completely smooth and incorporated.

Transfer batter to prepared pan; using spatula, spread batter into corners of pan and smooth surface. Sprinkle toasted nuts (if using) evening over batter and bake until toothpick or wooden skewer inserted into the center of the brownies comes out with few moist crumbs attached, 28 to 35 minutes. Cool on wire rack to room temperature, about 2 hours, remove brownies from pan, cut and serve. (Store leftovers in an airtight container at room temperature for up to 3 days).