Coat slow cooker generously with cooking spray
Whisk together flour, sugar, 4 tablespoons cocoa, baking powder, and salt
Stir in milk, oil, and vanilla until smooth
Fold in one bag of peanut butter chips
Spread in slow cooker
Mix brown sugar and remaining cocoa
Add hot water, stir until sugar dissolves
Pour over batter (don't stir!)
Top with remaining peanut butter chips
Cover and cook on HIGH 1½-2 hours until toothpick comes out clean
Turn off, remove lid
Let stand 30-40 minutes
Serve with sauce from bottom