Preheat oven to 325 degrees F (165 degrees C). Grease an 8x8 inch baking pan.
In a medium bowl, add cocoa to butter. Beat in sugar. Add eggs, vanilla, and salt and mix well. Stir flour into the mixture until just combined. Stir in walnuts if desired. Spread evenly into the prepared pan.
Bake for 25 to 30 minutes in the preheated oven, or until edges are firm. Cool before cutting into squares.