Preheat & Prepare: Heat oven to 425°F (218°C). Line a 12-well muffin tin with paper liners.

Combine Dry Goods: In a medium bowl, whisk all-purpose flour, cornstarch, baking powder, baking soda, and salt until uniform; set aside.

Mash Bananas: In a large bowl, mash overripe bananas with a fork into a pulpy texture with small fruit bits remaining.

Whisk Wet Base: Whisk melted butter, brown sugar, white sugar, eggs, and vanilla extract into the mashed bananas for 60 seconds until glossy and emulsified.

Fold Batter: Tip dry ingredients into the wet mixture. Use a silicone spatula to fold gently just until flour pockets disappear; fold in chopped walnuts if using. Do not overmix.

Portion: Divide batter equally among the 12 muffin cups, filling each nearly to the top rim. Sprinkle tops with turbinado sugar.

Two-Stage Bake: Bake at 425°F (218°C) for 5 minutes. Lower heat to 350°F (177°C) without opening the door; bake 15–17 minutes more until a toothpick inserted into the center comes out clean.

Cool: Rest in pan for 5 minutes, then move muffins to a wire rack to cool completely.