In microwave-safe bowl, melt 1 cup butter. Stir in sugar until dissolved. Add eggs and vanilla, stirring until blended.
Meanwhile, beat 1/2 cup butter until creamy. Combine confectioners' sugar and 2/3 cup cocoa powder in bowl and whisk until blended. Reduce mixer speed and gradually add confectioners’ sugar mixture until blended. Add milk and beat until creamy and smooth. Spread frosting over top of cooled brownie.
Makes 24 reindeer brownies.