Preheat oven to 350 degrees F. Line 8"x8"x2" baking pan with parchment paper and grease with butter. Combine beans,chocolate chips, flour,sugar,cocoa,baking powder,baking soda, salt,eggs,oil and vanilla extract in bowl of food processor.
Secure food processor lid and blend until mixture is smooth,about 2 minutes. Spread batter evenly into prepared baking pan, using rubber spatula to scrape batter from bowl. If desired,sprinkle chocolate chips and walnuts evenly over top.
Bake 30 minutes or until toothpick inserted in center of brownies comes out clean. Remove pan from oven and allow brownies to cool completely before cutting into squares.