Adjust the oven rack to the middle position and preheat the oven to 325°F (163°C).

Prepare your 13x9-inch baking pan by covering it with 2 folded sheets of aluminum foil, ensuring the foil overhangs by 3 inches.

Spray the foil-lined pan with nonstick cooking spray.

In a medium bowl, whisk together the flour, both salts, and baking powder.

Melt the butter in a large heatproof bowl and add the finely chopped chocolate until smooth.

Gradually whisk in the sugar, then add the eggs one at a time, mixing well after each addition.

Stir in the vanilla extract.

Gradually add the flour mixture in three parts, folding gently until the batter is smooth.

Pour the batter into the prepared pan, spreading it evenly and sprinkling nuts on top if using.

Bake for 30-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.

Let the brownies cool in the pan on a cooling rack. Lift them out using the foil overhang and cut into 2-inch squares.