Preheat oven to 350 degrees F.
In a mixing bowl, combine pureed beans, sugar, flour, cocoa and salt. Add oil, eggs, vanilla. Beat on low speed or by hand, scraping down sides of bowl, until smooth. Stir in walnuts (optional). Pour batter in a greased 9" x 13" pan, spread into corners and smooth top. Bake for 30 minutes or until tester inserted into center comes out clean. Cool in pan or wire rack. Cut into squares. After they're completely cooled cover brownies or put in an air tight container for at least a few hours.