Preheat oven to 350. Prepare baking sheets with cooking spray; set aside. In a mixing bowl, combine margarine, buttermilk, vanilla extract, and egg whites. In another mixing bowl, sift together flour, cocoa powder, sugar, baking soda, and salt. Mix wet ingredients with dry ingredients just until moistened. Drop dough by tablespoonfuls, 2" apart onto prepared baking sheets. Bake for seven minutes. Sprinkle with powdered sugar.