1In a bowl of a stand mixer, cream together 2.5 cups of sugar and 1 cup of unsalted butter at room temperature.
2In a small bowl, combine 1.25 cups of cocoa powder, 1 teaspoon of kosher salt, and 1 teaspoon of baking powder.
3Add the dry ingredients to the mixer and mix.
4Add four large eggs plus one large egg white to the bowl one at a time, making sure it's fully incorporated before adding the next egg.
5Add a half a cup of water.
6Beat on high speed until it's nice and creamy.
7Add 1.5 cups of all-purpose flour and mix minimally.
8Prepare the brownie pan by rubbing it down with vegetable oil and lining it with parchment paper.
9Pour the brownie batter into the pan and spread evenly.
10Place in a 350°F oven for 25–30 minutes, until it resembles a cake and a toothpick inserted emerges cleanly with a few dainty moist crumbs.
11Remove from oven and allow to cool completely on a wire rack for at least one hour.
12Meanwhile, make the frosting by creaming together 1 stick of softened butter, 2.5 cups of sifted powdered sugar, 0.5 cup of cocoa powder, and ⅓–⅔ cups of milk on high speed for about a minute, until finger-licking delicious.
13Once brownies have completely cooled, remove from parchment paper, frost liberally, and add sprinkles.