Preheat your oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.
In a large bowl, mash the bananas until mostly smooth.
Whisk in the eggs, yogurt, melted oil or butter, sugar, and vanilla until combined.
In a separate bowl, whisk the flour, oat flour, protein powder, baking soda, baking powder, salt, and cinnamon.
Add the dry ingredients to the wet and fold gently with a spatula. If the batter looks overly thick or dry, stir in up to 2 tablespoons milk. Do not overmix.
Fold in the chocolate chips, reserving a small handful for the top.
Scrape the batter into the prepared pan and smooth the top. Sprinkle on the remaining chips.
Bake for 45–58 minutes, or until a toothpick comes out with a few moist crumbs.
If the top browns too quickly, tent with foil in the last 10–15 minutes.
Cool in the pan for 10 minutes, then lift onto a wire rack to cool at least 45 minutes before slicing. This helps set the crumb and keeps it moist.