Heat oven to 350 degrees, convection if possible. Butter an 8-inch square pan, line the bottom with parchment and then butter the parchment (you can also use release foil).
In a double boiler over simmering water, melt the butter and chocolate. Remove the pan from the heat; cool slightly.
Stir in the sugar, salt, and vanilla. Temper the eggs by adding some of the chocolate mixture to the bowl with the eggs. Do this twice. Now add the egg mixture to the chocolate mixture, stirring until blended.
Add the flour and cocoa and beat until incorporated and the mixture is smooth, 30 to 60 seconds. (Add nuts if desired)
Scrape the batter into the prepared pan and bake until the top is flaky and a toothpick inserted in the middle comes out almost clean, with a few moist crumbs clinging to it, 20-22 minutes on convection or 25 to 30 minutes in a regular oven.
Set the pan on a rack and cool for about 10 minutes. If using release, you can simply lift the brownies out of the pan and place on rack to cool or just simply flip brownies onto a plate and flip again onto rack. Cool completely. Cut onto squares with a sharp knife.
Recipe can be doubled and baked in 13x9 pan and increase baking time slightly.