Combine flour, cocoa, sugar, Truvia, baking soda, and salt and mix well.
Stir in egg whites and vanilla. Fold in walnuts, or leave them out if you would like.
Spray a square baking pan (8x8 or 9x9) with cooking spray.
Spread batter evenly in the pan.
Bake at 350 degrees for 20 minutes or until the edges start to lift from the sides. Check the middle with a toothpick to be sure.
Slice and serve!