In a medium bowl, whisk together flour, cornstarch, cinnamon, baking powder, baking soda, and salt. Set aside.
1 ½ cups all-purpose flour, 4 teaspoons cornstarch, 1 teaspoon ground cinnamon, ½ teaspoon baking powder, ½ teaspoon baking soda, ¼ teaspoon fine sea salt
In a large bowl, whisk together eggs and brown sugar until foamy.
3 large eggs, ¾ cup brown sugar
Mash bananas (you should have about 1 cup) and add to the eggs along with the melted butter and vegetable oil. Mix well.
2 to 3 very ripe bananas, 4 tablespoons unsalted butter, 3 tablespoons vegetable oil
Add the dry ingredients to the wet ingredients and mix just until combined and no dry pockets remain. Do not overmix.
Lightly coat an 8x4-inch loaf pan with nonstick spray or butter. Add the sesame seeds, turning the pan carefully to stick the seeds to the bottom and up the sides.
3 tablespoons black sesame seeds
Pour in the batter, using a spatula to smooth the top and spread batter to the corners. Slice the remaining banana and arrange on top, then fill in the gaps with the walnuts.
1 ripe but firm banana, ⅓ cup roughly chopped walnuts
Bake for 60-65 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the pan for 5-10 minutes, then carefully turn out onto a wire rack to cool completely.
Notes
To quickly "ripen" bananas, bake in a 325°F until the peels are black and shiny. Let cool, then mash.
If you prefer, the walnuts and sliced banana can be folded into the batter just before pouring into the pan, rather than placed on top.
Make sure to let the loaf cool completely before slicing to prevent a gummy texture.