Place rack in the upper third of the oven and preheat the oven to 350 degrees F. Line a 12-cup muffin tin with paper or foil liners, or lightly grease with nonstick spray.
Mash bananas in the bottom of a large bowl until mostly smooth. Whisk in the egg, and then the yogurt and oil. Whisk in the brown sugar, maple syrup, and vanilla. Sprinkle the baking soda, salt, and cinnamon over the top, and then stir until combined. Gently stir in the flour until barely combined, and then fold in the walnuts.
Scoop the mixture into the prepared muffin tin, filling each cup nearly to the top. Bake for 20 to 22 minutes, until a toothpick inserted in the center comes out clean and the muffins spring back lightly when touched in the center.
TO STORE: Store muffins in a paper towel-lined airtight storage container at room temperature for up to 4 days.
TO FREEZE: Freeze muffins in an airtight freezer-safe storage container for up to 3 months. Thaw and enjoy as desired.