Lightly coat a 5-quart slow-cooker with the cooking spray, then line the bottom with parchment paper and lightly coat that in spray.
In a small bowl, whisk together flour, cocoa, baking powder, and salt.
Place butter and chocolate in a microwave-safe bowl and microwave in 30-second increments, stirring between each increment until the chocolate is melted. Add the sugar and and vanilla extract then stir to combine. Stir in the eggs, then add in the flour mixture, walnuts, and chocolate and stir until just moistened (be careful to not overmix). Transfer to a slow cooker and smooth out the top.
Cover and cook on low for 3 1/3 hours. Uncover and cook for another 30 minutes. Remove the ceramic insert from the slow cooker and run a knife around the edge to loosen brownies. Let the brownies cool completely in the insert on a wire rack (about 2 hours). Turn out onto a work surface and cut into brownies.