Step 1

Preheat the oven to 325 degrees F (163 degrees C). Line an 8 1/2 x 4 1/2 in (22x11 cm) loaf pan with parchment paper, with extra hanging over the sides for easy removal later.

Step 2

Combine the almond flour, coconut flour, baking powder, erythritol, xanthan gum, and sea salt in a large food processor. Pulse until combined.

Step 3

Add the melted butter. Pulse, scraping down the sides as needed, until crumbly.

Step 4

In a very large bowl, use a hand mixer to beat the egg whites and cream of tartar (if using), until stiff peaks form. Make sure the bowl is large enough because the whites will expand a lot.

Step 5

Add 1/2 of the stiff egg whites to the food processor. Pulse a few times until just combined. Do not over-mix!

Step 6

Carefully transfer the mixture from the food processor into the bowl with the egg whites, and gently fold until no streaks remain. Do not stir. Fold gently to keep the mixture as fluffy as possible.

Step 7

Transfer the batter to the lined loaf pan and smooth the top. Push the batter toward the center a bit to round the top.

Step 8

Bake for about 40 minutes, until the top is golden brown. Tent the top with aluminum foil and bake for another 30-45 minutes, until the top is firm and does not make a squishy sound when pressed. Internal temperature should be 200 degrees. Cool completely before removing from the pan and slicing.