Preheat oven to 350º.
Line an 8 x 8 pan with non-stick foil.
In a large microwave safe bowl, melt butter.
Stir in sugar, eggs, and vanilla. Mix in cocoa, flour, salt, and baking powder.
Spread batter into prepared pan.
Bake in preheated oven for 23 to 28 minutes or till toothpick inserted in center has just a few moist crumbs.
Do not overbake. Cool completely.
When brownies are cool, beat together frosting ingredients till well combined. Frost brownies.