In a large mixing bowl, combine almond flour, baking powder, and salt. Mix well to break up any lumps in the almond flour.
In a separate bowl, whisk together the egg yolks and melted butter until smooth. This mixture should look creamy.
In another clean bowl, whip the egg whites (with cream of tartar if you're using it) using a hand mixer or stand mixer until stiff peaks form. This can take several minutes, so be patient.
Gently fold the yolk-butter mixture into the dry ingredients until just combined. Then, delicately fold in the whipped egg whites. This step is crucial: you don't want to deflate the egg whites, so be gentle!
Pour the batter into the prepared loaf pan and smooth the top with a spatula. Bake for 25-30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Remove the bread from the oven and let it cool in the pan for about 10 minutes. After that, transfer it to a wire rack to cool completely.