Prep: Preheat oven to 350°F degrees. Spray an 8x8-inch baking dish (or a 9x13 baking dish, but note the shorter cooking time) with non-stick spray.
Dry Ingredients: In a large bowl, whisk together flour, sugar, cinnamon, baking powder, baking soda, and salt. Set aside.
Wet Ingredients: In a blender*, combine the bananas, sweet potato, eggs, vanilla, coconut oil, and yogurt. Blend until smooth. (If needed, add just a splash of milk to help the blender get moving.)
Finish Batter: Using a spatula to clean out the blender, add the wet ingredients to the dry ingredients. Stir gently with a spoon just until combined (do not over-mix). Batter will be thick. Fold in the chocolate chips. Spoon batter into prepared 8x8 inch pan and spread evenly using a spatula.
Bake: Bake for about 35-40 minutes (for a 9x12, bake 25-28 minutes). It's done when a toothpick inserted in center of cake comes out clean.