To prepare frosting place butter into a small saucepan over medium heat. Cook and stir until melted then add cocoa powder. Whisk until smooth. Remove from heat and transfer to a medium-large mixing bowl. Let chocolate cool for 10 minutes. When cooled slightly, add milk/vanilla and powdered sugar, alternating until all is used or frosting is to your desired conistency. Frost over cooled brownies then top with mini M&M’s. Serve room temperature.