Preheat the oven to 325 degrees.
Add the water to the bowl of your stand mixer and then add in the egg white powder, cream of tartar, salt. allulose and arrowroot, if using.
Start the mixer with the whisk attachment on low speed. Gradually increase the speed until full. Mix on full speed for 5 minutes.
Reduce speed to low, add in the butter powder and mix just until combined and no lumps are left.
Spread the batter into a large parchment paper lined loaf pan.
Bake in the preheated oven for 40-45 minutes (If doing 2 mini loaves cook for 35 minutes).
Remove from the oven and allow to cool in the pan on a wire rack for about 10 minutes.
Remove from the pan and allow to rest on the wire rack until completely cool to the touch.
Slice and enjoy.
Kudos to @Maria Emmerich, the godmother of the egg white bread craze! https://mariamindbodyhealth.com/the-easiest-protein-sparing-bread ***To avoid confusion please note that I do not follow Maria's version of the keto diet and this recipe may contain ingredients that are not on her approved foods list.***