These simple brownies are seriously tasty. They're fudgy with the perfect amount of frosting, but not too sweet.
Preheat oven to 350 F. Lightly spray a 9x13 cake pan with non-stick baking spray. In a medium-sized bowl, whisk together the flour, cocoa powder, baking powder, cornstarch, and salt. Set aside.
Add the granulated sugar, light brown sugar, and unsalted butter to the bowl of a stand mixer fitted with a whisk attachment. Cream together for a minute or two until the mixture is creamy. Scrape sides of the bowl. Add eggs, one at a time, scraping in between each addition. Add in vanilla and mix until combined.
Slowly add one third of the dry ingredients, followed by half of the milk. Scrape sides of the bowl down, then repeat. Lastly, add the last third of the dry mixture and mix until just combined, scraping down the sides of the bowl as you go. Scoop the batter into the baking pan and spread evenly. Bake for about 15 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Allow brownies to completely cool.
Cream the butter in the bowl of your mixer using the paddle attachment. Add powdered sugar then cocoa, then slowly mix on the lowest speed until the mixture is absorbed by the butter. Add vanilla extract, salt, and milk/cream and mix on medium speed until smooth, about 3 minutes. Frost evenly over the top of the brownies, decorating with sprinkles if you'd like. Enjoy!