Preheat oven to 350°F (175°C) and grease a 9x13 inch baking pan. In a medium bowl, whisk together flour, baking soda, salt, and cinnamon.

In a large bowl, cream softened butter with granulated and brown sugars until light and fluffy. Beat in eggs one at a time, then stir in mashed bananas, vanilla extract, and buttermilk.

Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined and no streaks of flour remain. Do not overmix.

Pour the batter into the prepared baking pan and spread evenly. Bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean.

Let the cake cool in the pan on a wire rack for at least 15 minutes before transferring to cool completely. Slice and serve plain or with your favorite frosting.