Heat oven to 350°F. Grease 9 x 13 baking pan or line with parchment paper..
Melt butter in microwave in large glass bowl.Stir in sugar and vanilla with a wooden spoonl.
Add eggs; beat well with spoon.
In separate bowl sift together flour, cocoa, baking powder and salt; gradually add to egg mixture, beating until well blended.
Spread batter evenly in prepared pan.
Bake 25 to 30 minutes or until brownie is set and the middle doesn't jiggle when pan is jostled. Cool completely in pan on wire rack.
Frosting: Beat first 4 ingredients together in a large bowl with electric mixer then slowly add in icing sugar. Then add milk until desired consistency is reached. Spread on cooled brownie.
Recipe by Chateau Elma at https://www.chateauelma.com/best-brownie-with-chocolate-frosting