Preheat oven to 350º. Spray a half-sheet pan with non-stick spray, line with parchment, and spray parchment. Or line the pan with non-stick foil (my preference).

Combine butter and chopped chocolate in a microwave-safe bowl. Microwave for 60 seconds, stop and stir, and continue microwaving for 30-second intervals, stopping and stirring till the mixture is smooth and melted. Set aside.

Whisk eggs in a large bowl and mix in salt, sugar, brown sugar, and vanilla. Stir in the chocolate mixture, then fold in the flour.

Pour batter into prepared pan and smooth top with an offset spatula.

Bake for about 45 minutes, until the top has formed a shiny crust. Cool in pan.

Cover with plastic wrap when cool and refrigerate for easy cutting.

To serve, lift the cold brownies out of the pan using the foil and place them on a cutting board. Peel back the foil, and cut to the desired size.