01 -
Grease a 9x5 loaf pan and line the bottom with parchment paper.
02 -
Preheat the oven to 350°F.
03 -
If the peanut butter is not already soft, gently warm it until easy to stir.
04 -
Stir together flour, protein powder, sugar, baking powder, salt, baking soda, and optional cinnamon in a large bowl.
05 -
Whisk in water, peanut butter, and egg until just evenly mixed.
06 -
Spread the batter evenly into the prepared loaf pan.
07 -
Bake for 45 minutes or until a toothpick inserted into the center comes out mostly clean.
08 -
Let cool before going around the sides of the pan with a knife and transferring the bread onto a serving plate.