Preheat oven to 350. Spray a 9x5" loaf pan with nonstick spray. Set aside.
Begin with banana bread. In a medium bowl, whisk together flour, cornstarch, baking soda, and baking powder. Set aside. In a small bowl or measuring cup, whisk together almond milk and apple cider vinegar. Let stand.
In a large bowl or stand mixer, beat together mashed bananas, applesauce, sugars and vanilla until well combined. Add half the dry ingredients, followed by almond milk mixture, then remaining dry ingredients. Mix until just combined.
Pour half the batter into prepared loaf pan. Pour 1 cup melted cookie butter on top in an even layer. Top with remaining batter.
To make the streusel, combine oats, brown sugar and 1 1/2 tbsp cookie butter. Mix by hand until ingredients are combined and form a crumbly texture. Sprinkle evenly over batter.
Bake until golden and set, and a toothpick inserted in the center comes out mostly clean, about an hour. Cool completely. Store leftovers in an airtight container for up to 1 week.