Prep the oven & pan: Preheat oven to 350°F (175°C). Lightly grease an 8x8-inch baking pan or line it with parchment paper.
Mix the wet ingredients: In a large mixing bowl, mash bananas until smooth. Whisk in sugar, oil, vinegar, and applesauce.
1 cup mashed bananas, ½ cup brown sugar, ⅓ cup vegetable oil, ⅓ cup applesauce, 1 tablespoon apple cider vinegar
Add dry ingredients: In a separate bowl, whisk together flour, baking powder, salt, and cinnamon.
1 cup all-purpose flour, ½ cup whole wheat flour, 1 teaspoon baking powder, ½ teaspoon salt, ½ teaspoon ground cinnamon
Combine: Stir the dry mixture into the wet mixture until just combined. Do not overmix.
Bake: Pour the batter into the prepared pan. Bake for 28–32 minutes, or until a toothpick inserted into the center comes out clean.
Cool & serve: Let cool in the pan for at least 10 minutes before slicing into squares. Enjoy warm or at room temperature.