Preheat your oven to 160 degrees Celsius (320 degrees Fahrenheit). Line a loaf pan measuring approximately 19 x 10 cm (7.5 x 4 inches) with parchment paper, ensuring some overhang on the sides for easy removal after baking.

In the bowl of a food processor, combine the almond flour, coconut flour, baking powder, optional sweetener, optional xanthan gum, and fine sea salt. Pulse several times until thoroughly combined.

Add the melted butter to the food processor. Pulse again, scraping down the sides as needed, until the mixture resembles coarse crumbs.

In a large, clean mixing bowl, using an electric hand mixer, beat the egg whites together with the optional cream of tartar (if using) and the optional vanilla extract until firm, glossy peaks form. Ensure your bowl is sufficiently large as the egg whites will expand significantly in volume.

Gently incorporate about half of the whipped egg whites into the food processor. Pulse briefly, just until the mixture is barely combined. Be careful not to over-mix.

Transfer the contents of the food processor to the bowl containing the remaining whipped egg whites. Using a spatula, gently fold the two mixtures together until no white streaks of egg white remain. Avoid stirring or vigorous mixing to maintain the airiness of the batter.

Pour the prepared batter into the parchment-lined loaf pan and smooth the top surface. You can gently push the batter slightly towards the center to encourage a rounded top during baking.

Bake in the preheated oven for approximately 45 minutes. At this point, the top should have a light golden-brown color. Tent the loaf loosely with aluminum foil and continue baking for an additional 40-45 minutes, or until the top feels firm to the touch and doesn't sound hollow or squishy when lightly pressed. The internal temperature should reach around 93-95 degrees Celsius (200-203 degrees Fahrenheit).

Allow the loaf to cool completely in the pan before lifting it out using the parchment paper overhang and slicing.