Preheat oven to 360°F. Grease and line a 9x5 inch loaf pan with parchment paper.
Make the streusel topping: In a medium bowl, combine the flour, brown sugar and cinnamon. Cut in the cold butter using a pastry cutter or forks until mixture is crumbly. Refrigerate while preparing the batter.
In a large bowl, whisk together the whole wheat flour, cornstarch, cinnamon, baking powder and salt.
In a separate bowl, whisk together the eggs, mashed bananas, granulated sugar, oil and Greek yogurt until well combined.
Pour the wet ingredients into the dry ingredients and fold together with a rubber spatula just until no dry streaks remain. Do not overmix.
Spread the batter into the prepared loaf pan and spread the top evenly. Sprinkle the chilled streusel topping evenly over the top, pressing gently to adhere.
Bake for 50-60 minutes, until a toothpick inserted in the center comes out clean or with just a few moist crumbs.
Allow the bread to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.