Preheat oven to 350 F.
Line a 9x5 inch loaf pan with parchment paper. (There are narrower pans on the market, that may produce a taller, longer bread loafs)
In a bowl combine coconut flour, almond flour, salt, baking powder and Xantan gum. Set aside.
Separate egg whites and egg yolks. Beat egg yolks for 3 minutes, then slowly add butter, beating to incorporate.
Beat egg whites with cream of tartar until stiff peaks.
Combine beaten egg yolks with butter with the flour mixture.
Add milk and beat to incorporate.
Fold in the egg whites into the mixture.
Transfer batter to the prepared pan and smooth with a spatula.
Bake for 50-60 minutes, until the bread is fully baked. Toothpick inserted in the middle should come out clean.
Let the bread cool inside the pan for 10 minutes, then take out. Cool completely.