Prep Your Oven & Pan: Preheat your oven to 350°F (175°C). Line a small loaf pan (an 8.5 x 4.5 inch loaf pan works great) with parchment paper, letting the edges hang over for easy lifting later. You can also grease the pan well if you don't have parchment.
Whisk the Dry Ingredients: In a medium bowl, whisk together the blanched almond flour, coconut flour, baking powder, sea salt, optional xanthan gum, and optional sweetener (if using a dry one). Give it a good mix to make sure everything is evenly distributed.
Melt the Butter: Melt your measured butter (or coconut oil) and set it aside to cool slightly. You don't want to add hot butter to your egg whites!
Whip Those Egg Whites: Grab a large, clean mixing bowl. Pour in your room temperature egg whites. If you're using cream of tartar, add it now. Using an electric mixer (handheld or stand mixer), whip the egg whites on medium-high speed until stiff peaks form. This means when you lift the beater, the peaks stand up straight and don't flop over. Be patient, this is important for the bread's structure!
Gently Fold in the Butter: With your mixer on low speed, or by hand using a spatula, gently drizzle the slightly cooled melted butter into the whipped egg whites. Fold it in carefully until just combined. We want to keep all that beautiful air in the egg whites!
Fold in the Dry Ingredients: Now, gently add the dry ingredient mixture to the egg white mixture in stages (maybe 2 or 3 additions). Use a spatula to gently fold the dry into the wet. Cut down into the center, then scoop up the sides and over. Do not stir or beat. We are aiming to combine everything without deflating the fluffy egg whites. Stop as soon as there are no more streaks of dry flour. A few small lumps are better than deflated batter!
Into the Pan & Bake: Carefully pour or spoon the batter into your prepared loaf pan. Smooth the top gently if needed.
Bake! Pop the pan into the preheated oven. Bake for 30-40 minutes, or until the bread is golden brown on top and a toothpick inserted into the center comes out clean. The top should spring back slightly when gently pressed.
Cool: Let the bread cool in the pan for about 10-15 minutes before carefully lifting it out using the parchment paper overhang (or inverting it onto a wire rack if you greased the pan). Let it cool completely on a wire rack before slicing. Slicing too early can make it seem gummy.
Enjoy! Once cool, slice it thick and enjoy it however you like!