Preheat oven to 350F. Butter and flour a large loaf pan. Set aside.
Make sure the bananas are thoroughly mashed with a fork, and then beat with an electric mixer until completely smooth. Add eggs, oil, maple syrup, sour cream and vanilla. Whip until just fully incorporated.
Measure the flour and mix the baking soda, salt and cinnamon into the flour. Add the flour mixture to the banana mixture and mix until just combined. Do not overwork the batter.
Pour the batter into the prepared pan. Bake for 40 to 45 minutes, until the tops are domed and completely set. They should spring back when pressed lightly with a finger (not sink in and dent). You can also test with a toothpick, which should come out clean or with a few tiny cooked pieces of bread attached.
Allow the bread to cool for at least 15 minutes in the pan before turning it out onto a cooling rack. Cool completely before cutting, at least 1 hour. Maple banana bread will keep for a week on the counter or up to 6 months tightly wrapped in the freezer.