Preheat the oven to 350°F. Grease a 9x13 baking pan or line with parchment paper. In a large mixing bowl, whisk together the butter, cocoa, brown sugar, and white sugar. Add the eggs and the vanilla. Whisk again until smooth. Add the flour and stir until combined.
Pour into the prepared pan and use a spatula to spread the thick batter to the corners. Bake for 22-24 minutes, until a toothpick comes out with moist crumbs.
Frosting Instructions
While the brownies are baking, make the frosting. In a mixing bowl, combine the butter, powdered sugar, cocoa, and salt. Beat with an electric mixer for 1-2 minutes, until combined. The mixture will be mostly dry crumbs. Add the milk and beat again for 1-2 minutes, until smooth and creamy.
Remove the brownies from the oven and let them cool for just a few minutes. Gently spread the frosting over the brownies while they are still quite warm. Let cool before slicing.
Notes
Frost the brownies while they are still warm. The frosting will melt down a bit into the brownies creating an irresistible crust.