Preheat oven to 350F. Coat a 13x9x2-inch baking pan with nonstick cooking spray and dust with flour.
To prepare the brownies: In a mixing bowl, beat margarine until fluffy. Gradually add sugar and beat well. Add egg whites, sour cream, milk and vanilla; beat well.
Combine flour, baking powder, salt, if using, and cocoa in another bowl. Add flour mixture to creamed mixture, mixing well.
Pour batter into the baking pan and bake 25 mintues, or until a wooden pick inserted in the center comes out clean. Let cool in pan on a wire rack.
To prepare the frosting: In a mixing bowl, blend confectioners' sugar, cocoa, vanilla and skim milk until frosting is of spreading consistency. Spread over top of cooled brownies and cut into squares.