Heat oven to 350°F (180°C) and fold a 7 x 7 inch (15 x15 cm) brownie pan with parchment paper.
In a large bowl mash the bananas and combine with yogurt, vanilla extract, cinnamon and a pinch of salt. For sweetener bars, add 2 to 4 tablespoons of maple syrup or honey to taste. Stir until smooth.
Add rolled oats and oat flour (or protein powder) to the wet mixture and stir until combined. You can optionally add extra mix-ins, such as chocolate chips, walnuts or raisins.
Transfer the dough to the prepared baking pan and spread it out evenly.
Bake in hot oven for 15 to 20 minutes, or until they have set in the center. Let cool to temperature, then slice into bars.