Preheat oven to 325 degrees F (165 degrees C). Grease a 15x10 inch baking pan.
Combine the margarine and the sugar in a large mixing bowl; use an electric mixer to beat until light. Beat in the eggs and vanilla; mix well. Whisk together the flour, cocoa powder, walnuts, and salt in a large bowl. Stir the flour mixture into the egg mixture, mixing just until dry ingredients are incorporated. Spoon batter into prepared pan.
Bake brownies in the preheated oven until the edges begin to pull away from the sides of the pan, 40 to 45 minutes.