Grease the sides of your baking dish and line it with a piece of parchment paper. (This prevents the batter from sticking to the bottom and makes it easy to remove them from the pan)
Preheat oven to 350 °F. (175 C)
Using a sifter or sieve, sift the King Arthur Keto Flour, salt and cocoa powder.
In a large mixing bowl, cream the butter.
Add the eggs, egg wgites and vanilla extract and continue mixing.
Add the sweetener and mix.
Gradually mix in the dry ingredients until the batter is smooth and there are no lumps.
Pour the batter into a parchment lined baking pan.
Bake in the oven for approximately 35 minutes. or until a toothpick comes out clean.
Remove from oven and allow to cool before removing from the pan.