Whisk brownie box mix with eggs, water and vegetable oil until smooth.
Spread out evenly in a parchment lined half-sized sheet pan (13x18)
Bake in a 350 degree preheated oven in the middle rack for 18-22 minutes or until a toothpick comes out clean.
Allow to cool for at least an hour.
To a medium sized pot melt together the butter, peanut butter and milk over medium heat.
Once smooth add powdered sugar one cup at a time and whisk until there are no clumps.
Add the vanilla and whisk once more.
While hot, pour the icing over the brownies and immediately spread out.
The icing will begin to harden so work quickly.
Cool for two hours. Slice and serve.