Whip the egg whites and cream of tartar (I like to use a whip attachment in a stand mixer) until stiff peaks form. This is the most important step for making sure your bread becomes fluffy and tall and not dense and flat.
Mix the egg yolks, melted butter, almond flour, coconut flour, stevia, and sea salt in a separate bowl until well combined. If it is too thick to mix well, add a little bit of the egg white (before whipping) to it until you can easily mix it.
Pour batter into prepared loaf pan and smooth with a spatula. Batter should come up to about 1 inch below the top of the loaf pan; if there is extra, bake in a small custard cup or muffin pan. Sprinkle the top if desired with extra almond flour. We just sprinkle the top with flour to give it a more 'bread like' appearance.
Standard-sized metal loaf pan is used for this recipe, I use this one.
Nutrition facts assume 10 slices of bread are cut per loaf.