Preheat the oven to 350°F (175°C). Grease and lightly flour an 8.5 x 4.5-inch loaf pan and set aside.
Peel and mash 2 to 3 very ripe bananas with a mixer until smooth and applesauce-like, then measure 1 cup and set aside.
Place about a heaping 1/3 cup oats in a blender or food processor and blend until fine to make oat flour; measure a leveled 1/3 cup.
In a large bowl, whisk together the mashed banana, 1/2 cup vanilla Greek yogurt, 1 large egg, 1 1/2 teaspoons vanilla extract, 5 tablespoons melted (but not hot) coconut oil, 2 tablespoons honey, and 1/2 cup lightly packed light brown sugar until smooth.
Add 1 teaspoon baking soda, 1/4 teaspoon salt, 1 teaspoon ground cinnamon, 1 tablespoon cornstarch, and the 1/3 cup oat flour on top of the wet mixture and whisk to combine.
Add 1 cup all-purpose flour and stir, then use an electric hand mixer briefly and beat just until combined; do not overmix.
Pour the batter into the prepared loaf pan. If desired, sprinkle 2 tablespoons brown sugar evenly over the top for a sweeter crust.
Bake for 45–60 minutes, until a fork or toothpick inserted in the center comes out clean or the bread pulls away from the edges; if the top browns too quickly, tent loosely with foil.
Let the bread cool in the pan for 10–15 minutes, then transfer to a wire rack to cool before slicing.
