Preheat the oven to 350°F (175°C). Grease and flour a 9×13 inch (23×33 cm) baking pan.
In a medium bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon. Set aside.
In a large bowl, cream the softened unsalted butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time to the butter and sugar mixture, beating well after each addition.
Mix in the vanilla extract.
In a separate bowl, combine the sour cream, mashed bananas, and buttermilk.
Add the dry ingredients to the wet ingredients in three additions, alternating with the banana mixture, beginning and ending with the dry ingredients. Mix each addition just until combined; do not overmix.
Pour the batter evenly into the prepared baking pan and smooth the top with a spatula.
Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan on a wire rack for 15 minutes.
After 15 minutes, remove the cake from the pan and allow it to cool completely on the wire rack before serving.