Preheat oven to 325°. Line the cupcake pan with the cupcake liners; set aside.
In a large bowl, whisk together eggs,1⁄3 cup of vegetable oil and 1 tablespoon of water.
Add the brownie mix to the large bowl and whisk until well combined.
Spoon the batter into the cupcake liners until about 3⁄4 full. Bake for 20 to 25 minutes or until a toothpick or fork inserted in the middle of the cupcake comes out clean. Allow cupcakes to cool completely before frosting.
While cupcakes are cooling, prepare the frosting. In a standing mixer or with a hand mixer on high speed, mix together the butter and peanut butter until smooth. Slowly add in the vanilla frosting mix and 2 tablespoons of milk. Whip on medium-high speed until smooth and well combined.
Use a knife (or spatula) to frost the brownie cupcakes. Enjoy!