Set the oven to 350°, and butter a square baking pan
Melt the butter, and mix in xylitol and blend until it dissolves
Add the egg yolks to the butter
Mix the flours, salt, baking soda, and baking powder together
Add the combined dry ingredients to the yolks
Add in the liquid ingredients, and stir till smooth
Whip the egg whites with cream of tartar until it forms stiff peaks
Mix a little of the whipped egg whites into the yolk mixture to lighten it. Then gently fold the yolk mixture into the egg whites, being careful not to deflate the whites too much.
Spread the batter into the baking pan, and bake for 35 minutes
Allow it to cool before cutting it into square