Cut cold butter into flour, brown sugar, and cinnamon until it forms a crumbly mixture. Chill while preparing the batter; this ensures a crisp, flavorful topping.
In a large bowl, whisk together whole wheat flour, cornstarch, baking powder, and salt. This combination creates a light, fluffy base that complements the moist bananas.
Mash bananas in a separate bowl, then stir in eggs, Greek yogurt, oil, and sugar. Mixing these ingredients well ensures a consistent texture throughout the bread.
Gently fold the wet ingredients into the dry mix using a spatula. Stop once combined to avoid overmixing, which can make the bread dense.
Line a loaf pan with parchment paper, pour in the batter, and evenly spread the cinnamon streusel on top. This step guarantees a perfect crunch with every bite.
Bake at 350°F (175°C) for about 60 minutes, or until a toothpick inserted in the center comes out clean. Let it cool in the pan for 10 minutes before transferring to a wire rack.