Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x11 inch baking dish.
In a large bowl, mix the butter and brown sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla extract. Mix in the flour, salt and cocoa powder just until moistened. Stir in the walnuts, if using. Spread the batter evenly into the prepared pan.
Bake for 20 minutes in the preheated oven, or until the top is dry and the edges begin to pull away from the sides of the pan. Cool before cutting into squares.