Coat a medium round or oval slow cooker with nonstick cooking spray.
In a medium size bowl, whisk together the flour, cocoa, sugar, baking powder, and salt. Make a well in the center, add the milk, oil, and vanilla, and stir in widening circles, gradually incorporating the dry ingredients, until you have a smooth batter. Stir in the peanut butter. The batter will be thick. Stir in the chocolate chips. Spread evenly in the crockpot.
To make the topping, in another medium-size bowl, combine the cocoa, and sugar; pour in the boiling water and whisk until smooth. Gently pour over the batter in the cooker; DO NOT STIR. Cover and cook on HIGH until puffed and the top layer is set, 2 to 2 ½ hours.
Turn off the cooker and let it stand, covered for 30 minutes before serving.
Serve with vanilla ice cream.
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