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 over-mix). Transfer to slow cooker and smooth top.
Cover and cook on LOW for 3 1/2 hours. Uncover and cook 20 minutes more. Remove insert from slow cooker and run a knife around edge to loosen brownies. Let cool completely in the insert on a wire rack, about 2 hours. Turn out onto a work surface and cut into desired shapes.
Cook's Note: The center may look undercooked when time is up, but it will be perfect once cooled