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Preheat oven to 350 degrees and grease a 10” springform pan (a 9” pan will work as well; the brownie will be slightly thicker).
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Mix brownie mix, butter, egg, oil, water, and vanilla extract until smooth.
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Pour the batter into the pan and spread in an even layer.
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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.
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While the brownie pizza is cooling, make the icing.
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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.
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Add in vanilla extract, salt, and 4 Tbsp of milk. Mix.
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If the peanut butter icing is too thick, add in another Tbsp of milk.
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Spread peanut butter icing over the brownie pizza and top with chopped peanuts, chocolate chips, and peanut butter chips.
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Allow the icing to set before slicing into the pizza (about 10 minutes or so).
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Enjoy!