Melt butter in a sauce pan over medium heat. Add sugar, cocoa and milk. Whisk to combine and continue whisking until the mixture is smooth and there are no lumps.
Boil for 1 minute, while continuing to stir or whisk. Remove from heat. Add vanilla and stir to incorporate.
Remove from heat. Add vanilla.
Allow to cool for about 5 minutes and pour icing over brownies.
Allow the icing to cool completely and set before slicing brownies.
Notes
Makes enough icing for 9 brownies, cooked in an 8X8 pan.
For extra gooey brownies, you may poke holes in the brownies before pouring on the icing.
To speed up setting and cooling process, place iced brownies in the refrigerator.
The icing will set up quicker if you allow the brownies to cool completely before pouring it on.
For a thicker, sweeter icing, increase powdered sugar to 2 cups. Alternately, you can use 1/2 stick of butter with 1 1/2 cups powdered sugar.