Preheat your oven to 375°F. Grease and line a 9 inch x 5 inch loaf pan and set it aside.

In the bowl of a stand mixer fitted with the whisk attachment, beat 2 of the egg whites on high until stiff (but not dry) peaks form, adding the cream of tartar about halfway through.

With a silicone spatula, gently scrape the beaten egg whites into a medium-sized bowl and set the bowl aside.

In a separate, medium-size bowl, place the flour, xanthan gum, salt, whey powder and baking powder and whisk to combine.

To the bowl of the stand mixer fitted with the paddle attachment, place the butter, honey, vinegar, the 2 remaining egg whites, and milk, then top with the dry ingredients and mix to combine well. The batter will be stiff and thick.

Add one-third of the whipped egg whites to the dough, and slowly mix using the paddle attachment until the egg whites are incorporated into the batter.

Remove the bowl from the mixer, and gently fold the remaining egg whites into the batter until nearly no white streaks remain.

Scrape the dough into the prepared loaf pan.

Smooth the top of the dough in the pan with a wet spatula.

Grease the underside of a piece of aluminum foil and tent the loaf pan with it.

Place the loaf pan in the center of the preheated oven, and bake for about 50 to 60 minutes (rotating once during baking).

Remove the foil and continue to bake until the top is brown and a toothpick inserted into the center comes out clean.

The bread will rise higher than you expect and will fall as it cools.

Allow the bread to cool for a few minutes in the pan, and then turn out onto a wire rack to cool completely before slicing and serving.