Spray your slow cooker with non stick cooking spray.
In a separate bowl add flour, cocoa powder, baking powder, salt and sugar. Mix to combine.
Make a well in the centre and add vegetable oil, milk and vanilla extract. With a wooden spoon, stir until well combined and a smooth mixture is produced.
Add peanut butter and stir through. Note: this will be a thick batter and difficult to stir.
Add chocolate bits and mix through.
Spread mixture evenly in the slow cooker.
Chocolate Topping
In a medium bowl add cocoa and sugar and stir to combine.
Add boiling water and stir until completely combined.
Pour topping over the batter in the slow cooker, but do not stir through.
Cover the slow cooker, set to high and cooker for 2 1/2 hours.
Once done, turn off slow cooker and let stand, while still covered in the slow cooker, for 30 minutes before serving.