Preheat the oven to 350°F (175°C) and lightly grease a 12-cup muffin pan. This ensures that the muffins bake evenly and don’t stick to the pan, making them easier to remove once they’re done. Greasing the pan helps achieve a golden, crisp exterior while maintaining a soft interior.
In a large bowl, combine the whole wheat flour, brown sugar, cinnamon, baking powder, and baking soda. In a separate bowl, mix the mashed bananas, egg whites, and vanilla extract. This separates the dry and wet ingredients, which helps ensure that the dry ingredients are evenly distributed throughout the batter while the wet ingredients are smoothly incorporated into the mix.
Stir the banana mixture into the flour mixture until smooth and well combined. Gently fold in the blueberries to avoid mashing them. Spoon the batter into the prepared muffin pan, filling each cup about two-thirds full. This ensures that the muffins bake evenly and rise to a perfect texture, while the blueberries are evenly distributed throughout the batter.
Bake for 16 minutes in the preheated oven, or until a toothpick inserted into the center of a muffin comes out clean. This ensures that the muffins are fully cooked and have a light, fluffy texture, without being overbaked or underdone.