Preheat oven to 350°F. Grease and line a 8 inch square pan with parchment paper for easy removal and clean up.
In a large bowl, combine all the dry ingredients.
In a medium bowl, mash bananas in a large bowl using the back of a fork or a potato masher. Try to get it as smooth as possible but small lumps are fine. Measure out 1 cup. To the same bowl, add all the wet ingredients and mix together until well combined.
Stir the wet ingredients into the flour mixture. Fold in the walnuts.
Pour the batter into the prepared pan and bake for 25-30 minutes, or until the inserted toothpick in the center comes out clean. Transfer to a wire rack and allow to cool completely.
Optional: Spread cream cheese frosting on the cooled cake. Refrigerate for 30 minutes before serving to help the frosting to set and make cutting easier.
Notes
Be sure to measure out the mashed bananas so you have exactly 1 cup. Too much banana can make the cake overly moist and soggy.
I've provided a range for the maple syrup amount so you can decide how sweet you want the cake to be. You can also substitute with date syrup, a wonderful alternative to added sugars, or leave out completely.
Store leftovers in an airtight container and refrigerate for up to 5 days.