Preheat oven to 350 degrees and grease a 10” springform pan (a 9” pan will work as well; the brownie will be slightly thicker).
Mix brownie mix, butter, egg, oil, water, and vanilla extract until smooth.
Pour the batter into the pan and spread in an even layer.
Bake for about 20 minutes, or until a tooth pick or cake tester comes out clean with just a few fudgy crumbs stuck to it.
While the brownie pizza is cooling, make the icing.
With a standing mixer or hand held mixer, mix melted butter and peanut butter together until well combined. Add in powdered sugar, 1/2 cup at a time, mixing well after each addition.
Add in vanilla extract, salt, and 4 Tbsp of milk. Mix.
If the peanut butter icing is too thick, add in another Tbsp of milk.
Spread peanut butter icing over the brownie pizza and top with chopped peanuts, chocolate chips, and peanut butter chips.
Allow the icing to set before slicing into the pizza (about 10 minutes or so).
Enjoy!