Prep the pan and oven. Heat oven to 350°F (175°C).
Line a 9x5-inch loaf pan with parchment paper, leaving overhang for easy lifting. Lightly grease the sides.
Mash the bananas. In a large bowl, mash the bananas until mostly smooth with a few small lumps. You should have about 1 1/4 to 1 1/2 cups.
Whisk in the wet ingredients. Add eggs, Greek yogurt, maple syrup or honey, vanilla, and melted oil.
Whisk until well combined and glossy.
Mix the dry ingredients. In a separate bowl, stir together the flour, protein powder, baking soda, baking powder, salt, and cinnamon. Break up any clumps, especially in the protein powder.
Combine wet and dry. Add the dry mixture to the banana mixture. Stir gently with a spatula until just combined.
If the batter seems too thick (like cookie dough), fold in 2–3 tablespoons milk to reach a thick, spoonable batter.
Add mix-ins. Fold in chopped nuts or chocolate chips if using, keeping the batter tender by not overmixing.
Fill the pan. Scrape the batter into the prepared loaf pan and smooth the top. For a prettier loaf, run a knife down the center to create a slight ridge.
Bake. Bake for 45–55 minutes, or until the top is set and a toothpick inserted in the center comes out mostly clean with a few moist crumbs. If the top browns too quickly, tent with foil for the last 10–15 minutes.
Cool completely. Let the loaf cool in the pan for 10 minutes, then lift it out and finish cooling on a wire rack.
This helps it set and slice cleanly.
Slice and serve. Cut into thick slices. Spread with 1–2 tablespoons nut butter per slice for a satisfying, protein-rich snack.