Brownie: In a mixing bowl, with an electric mixer, mix together the butter, sugar, cocoa and vanilla until completely smooth. Add the remaining ingredients and spread into a greased 9x13 pan. Bake at 350 degrees for 25-30 minutes (don't over bake but make sure the center is cooked through). Cool the brownies on the counter and then frost with chocolate frosting.
Frosting: In a mixing bowl, with an electric mixer, blend together the butter, cocoa powder, vanilla and milk until smooth and soft. Add the powdered sugar and dash of salt and continue to mix on medium-high speed until the frosting is light and fluffy, about 4-6 minutes. The longer you whip the frosting, the smoother and fluffier it will be. If your frosting is too thick, add a few teaspoons of milk or if it's too runny, add a little more powdered sugar. Spread over the cooled brownies.
Notes
HEALTHIER OPTION: Use half whole wheat flour instead of white in the brownies. Instead of serving with frosting, just sprinkle the top with powdered sugar.