This is more 'icing' than 'frosting'. Good for those who like a thin chocolate glaze on their brownie rather than thick fluffy frosting (or if you are out of powdered sugar and need something to coat your brownies). If you are looking for the thick fluffy frosting, try Chocolate Buttercream Frosting or Double Chocolate Buttercream Frosting.
1 cup granulated sugar
1/4 cup cocoa
1/4 cup butter
1/4 cup milk
1 teaspoon vanilla extract
Mix together sugar, cocoa, butter and milk in sauce pan. Bring to a boil. Stir and boil for 1 minute.
Turn off heat and add vanilla.
Cool partially, then beat with a mixer for 3 minutes or until spreadable consistency.
Pour over cooled brownies in brownie pan.