Start by preheating your oven to 350°F and prepare your bread pan by greasing it thoroughly or lining it with parchment paper. This ensures your bread won't stick and gives you time for the oven to reach temperature while you prepare the batter. If you're making cupcakes instead, prepare a muffin tin with liners.

In a large mixing bowl, whisk together the protein powder, flour, sweetener, cinnamon, baking soda, and salt until evenly combined. Whisking the dry ingredients ensures the leavening agents and spices are distributed uniformly throughout the batter, which prevents dense pockets in your finished bread and guarantees consistent flavor in every slice.

In a separate bowl, whisk together the mashed bananas, Greek yogurt, egg whites, peanut butter, and vanilla extract until smooth and well combined. Make sure the peanut butter is fully incorporated into the wet mixture—I like to give it an extra 30 seconds of vigorous whisking to ensure there are no peanut butter clumps, which helps create a smooth, evenly-textured batter.

Pour the wet ingredient mixture from Step 3 into the bowl of dry ingredients from Step 2. Fold everything together gently until just combined—don't overmix. Stop as soon as you no longer see streaks of dry ingredients. Overmixing develops the gluten in the flour, which can make your bread tough and dense instead of moist and tender.

Pour the batter into your prepared pan from Step 1, smoothing the top gently with a spatula. Place in the preheated 350°F oven and bake for 35-45 minutes (or 25-35 minutes if making cupcakes), until a toothpick inserted into the center comes out clean or with just a few moist crumbs. The bread is done when it springs back slightly when lightly touched on top.

Remove the bread from the oven and let it cool in the pan for about 10 minutes, then turn it out onto a wire rack to cool completely. Once cooled, slice and enjoy as-is, or spread a thin layer of additional peanut butter on top of each slice for extra richness if desired.