Lightly coat 5 quart slow cooker with cooking spray. Line bottom with parchment paper and lightly coat with spray. In a small bowl, whisk together flour, cocoa, baking powder and salt.
Place butter and chocolate in medium microwavable safe bowl and microwave in 30 second increments, stirring after each, until chocolate is melted. Add sugar; stir to combine. Stir n eggs. Add flour mixture, walnuts and chocolate chips and stir just until moistened (do not overmix). Transfer to slow cooker and smooth top.
Cover and cook on low for 3 1/2 hours. Uncover and cook for 30 minutes. Remove insert from slow cooker and run a knife around edges to loosen brownies. Let cool completely in insert on a wire rack, for 2 hours. Turn out onto a work surface to cut.