Prepare brownie mixes according to package directions using a 13x9-inch baking dish.
When done, remove from oven; set aside and let cool completely.
Melt margarine or butter in microwave. Stir in cocoa powder and milk until smooth.
Slowly add powdered sugar into butter mixture, stirring until smooth. Add vanilla extract, and mix until no lumps remain.
Spread frosting over brownies, and cut into squares.