In a medium sized bowl, add egg whites, sugar, vanilla, cocoa powder, baking powder, salt and flour. Stir so batter is completely combined. Add walnuts and stir with a spoon.
Spray a 9x13 inch baking dish with cooking spray. Spread batter into the dish. The batter will look like there’s not enough, but trust me, it’ll work.
Add marshmallow fluff on top of the batter and using a spatula, swirl around.
Bake for 20-22 minutes, or until toothpick comes out clean.
Allow to cool for 30 minutes before cutting and removing from dish.