Add the mashed bananas, softened butter, granulated sugar, and egg to the bread machine pan.
In a separate bowl, whisk together the flour, baking soda, and cinnamon.
Add the dry ingredients to the bread machine pan.
Add the vanilla extract to the bread machine pan.
Set the bread machine to the "sweet bread" or "quick bread" setting and start the machine. (I use "sweet bread" function)
Let the bread machine knead the dough and bake the banana bread until it's finished, My Oster Bread Maker takes about 3 hours.
Once the banana bread is finished, let it cool in the bread machine pan for 5-10 minutes before removing it and placing it on a wire rack to cool completely.
Slice and serve warm or at room temperature.