Mix together all ingredients but powdered sugar and milk. Mix the flour, cocoa, sugar, and salt; using a pastry blender or 2 knives, cut in margarine to form coarse crumbs. Blend in egg whites and vanilla, to make a soft dough.
Put dough in the bottom of a greased baking sheet. Spread dough evenly to cover whole pan. Poke dough with fork in a few places.
Bake 20 to 25 minutes. Let cool on wire rack. Drizzle glaze over top and slice into squares.
To make the sugar glaze whisk together powdered sugar and milk to form a spreadable glaze.