Preheat the oven to 325°F (163°C). Line a small to medium loaf pan with parchment paper, leaving extra hanging over the sides for easy removal.
In a food processor, combine the almond flour, coconut flour, baking powder, sweetener, xanthan gum, and salt. Pulse until well combined.
Add the melted butter and pulse again, scraping down the sides as needed, until the mixture is crumbly.
In a large bowl, beat the egg whites and cream of tartar (if using) with a hand mixer until stiff peaks form.
Add half of the whipped egg whites into the food processor mixture and pulse briefly until just combined. Don't overmix.
Transfer this mixture into the bowl with the remaining egg whites and gently fold together until no streaks remain. Fold, don't stir.
Spread the batter into the lined loaf pan and smooth the top, rounding it slightly.
Bake for about 40 minutes, until golden on top. Tent loosely with foil and continue baking for another 30 to 45 minutes, until firm and no longer squishy when pressed (internal temperature around 200°F). Cool completely before slicing.