Begin by preheating your oven to 350°F (175°C). This ensures that your brownies start baking at the right temperature for the ideal texture.
Grease your 9x13 inch baking pan or line it with parchment paper, leaving some overhang for easy brownie removal later.
In a medium bowl, whisk together the cake flour, kosher salt, coarse salt (if using), and baking powder. Set this mixture aside.
In a large mixing bowl, combine the chopped unsweetened chocolate and unsalted butter. Use a microwave or a double boiler to melt them together until smooth. Stir occasionally to avoid burning. Allow the mixture to cool slightly.
Once the chocolate mixture has cooled, whisk in the granulated sugar until well combined. Next, add the eggs one at a time, mixing well after each addition. Finally, stir in the vanilla extract.
Gently fold the dry ingredients into the wet ingredients using a rubber spatula. Be careful not to overmix; stop as soon as you see no more flour streaks.
If you're using pecans or walnuts, gently fold them into the brownie batter now.
Pour the batter into the prepared baking pan, spreading it evenly. Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan on a cooling rack for at least 15 minutes. Use the parchment paper overhang to lift them out and place them on the rack to cool completely before cutting into squares.