Prepare Your Oven and Muffin Tin: Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease with nonstick spray.
Mix the Wet Ingredients: In a large bowl, mash the ripe bananas until smooth. Add the egg, Greek yogurt, and oil, whisking until combined. Stir in the brown sugar, maple syrup, and vanilla extract until fully incorporated.
Combine the Dry Ingredients: Sprinkle the baking soda, salt, and cinnamon over the wet mixture. Gently stir to combine.
Add the Flour and Walnuts: Gradually fold in the whole wheat flour, mixing until just combined. Be careful not to overmix, as this can result in dense muffins. If using, gently stir in the toasted walnuts.
Fill the Muffin Tin: Divide the batter evenly among the 12 muffin cups, filling each nearly to the top.
Bake the Muffins: Bake for 20-22 minutes, or until a toothpick inserted into the center comes out clean and the tops are golden brown. Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely