Spread into a greased half sheet pan (18x13) and bake at 350 for about 25 minutes, or until set in the center. Allow the brownies to cool slightly before icing.
To make chocolate frosting
Place all of the ingredients in a mixing bowl and beat with an electric mixer until smooth and creamy. Spread onto the cooled brownies.
2/3 cup cocoa powder, 1/2 cup butter, 1/2 cup milk, 1 teaspoon vanilla extract, 4 cups confectioner's sugar