Preheat the oven to 350F. Butter a 9-by-13-inch glass baking dish.

In a small saucepan, melt the butter over low heat. Remove from the heat and add the chocolate. If the heat from the butter does not fully melt the chocolate, put the pan back over the heat for 10 seconds and stir until melted. Set aside to cool.

Sift the flour into a small mixing bowl. Set aside. In a medium mixing bowl, combine the eggs, sugar, salt, and vanilla. Using a mixer fitted with the whisk attachment, beat on high speed until the mixture thickens and becomes pale in color and falls from the beater in a wide ribbon that folds back on itself and slowly dissolves on the surface, 4 to 5 minutes. Using a rubber spatula, fold the cooled chocolate into the egg mixture. Add the flour and fold it in quickly but gently with the rubber spatula so that you don't deflate the air that's been incorporated into the eggs.

Pour the batter into the prepared dish and smooth the top with a spatula. If you are using nuts, evenly distribute them across the batter. Bake until the top looks slightly cracked and feels soft to the touch, about 25 minutes. Let cool completely on a wire rack.

Using a sharp knife, cut into 12 squares, or size desired. The brownies will keep in an airtight container in a cool place for up to 1 week.