Preheat oven to 165C/325F/Gas 3 and grease a 9x13 baking dish with butter or baking spray (or line with parchment paper).
Place brownie mix in a bowl with pumpkin, oil and water and stir until well combined.
Add brownie batter to the pan and spread evenly.
Make the peanut butter cookie dough according to package instructions.
With your hands, press out cookie dough and lay over the brownie layer until you have used all of the cookie dough (It will create a few gaps and that is okay).
Sprinkle with the Heath Toffee Bits and Reese's Pieces.
Bake for 40-45 minutes. Check brownie is cooked through by inserting a cocktail stick into the middle - it will come out clean when done.
Remove from oven and let the brownies cool completely.