Grate or shred the 1/2 cup of frozen butter and transfer it back to the freezer until time to add it to the batter.
Add in the egg white powder, cream of tartar, salt, allulose and arrowroot, if using.
Reduce speed to low and stir in the frozen butter shreds and whole egg or yolk powder 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 45 minutes.
Remove the bread from the pan and peel away the parchment paper.
Allow to rest on the wire rack until completely cool to the touch.
Substitution options:
1 tsp of gelatin can be replaced with 1/4 tsp of xanthan gum.
Allulose can be reduced. If you eliminate it all together try increasing the oven temp to 375-400 degrees in order to get some browning on your loaf. Check for doneness before the whole 45 minutes are up.
Arrowroot can be reduced to 1 tbsp or eliminated for a lower carb count.