in a large bowl, whisk together flour, sugar, 1/3 cup cocoa powder, baking powder, salt. Mix in milk, oil and vanilla extract until batter is smooth. Stir in nuts and chocolate pieces.
Coat the inside of a 3 1/2- to 4-quart slow cooker with nonstick cooking spray and spoon in batter.
In a medium bowl, whisk together brown sugar, 1/4 cup cocoa powder, and hot water. Gently pour over batter. Do NOT stir.
Cover and cook on high-heat setting for 2 hours.
Remove the liner from slow cooker, take off lid and let cool for at least 30 minutes. Serve warm.