Preheat the oven to 350F
Grease a 8" x 8" casserole dish and set aside.
In a large bowl, beat together the butter and sugar until combined.
Add the eggs and vanilla extract and beat until combined.
In a medium-sized bowl, stir together the cocoa powder, flour, salt and baking powder.
Beat the flour mixture into the sugar mixture.
Fold the brownie batter out into the prepared casserole dish.
Bake for 25-28 minutes, until the edges of the brownies are crisp and start to pull away from the pan and an inserted toothpick comes out with just a few crumbs and no wet batter.
For the brownie icing, combine all ingredients and beat together with an electric mixer. Adjust with additional milk or icing sugar to achieve your desired texture and flavor.
Once the brownies have cooled down, spread the browning icing all over and top with patriotic M&Ms. Cut into squares and serve.