Lightly coat a 5-quart slow-cooker insert 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 a medium microwave-safe bowl and microwave in 30-second increments, stirring after each, until chocolate is melted. Add sugar; stir to combine. Stir in 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, 3 1/2 hours. Uncover and cook 30 minutes. Remove insert from slow cooker and run a knife around edge to loosen brownies. Let cool completely in insert on a wire rack, about 2 hours. Turn out onto a work surface and cut into 14 brownies.
The center may look undercooked when time is up, but it will be perfect once cooled.