Preheat the oven to 350°F (175°C). Grease and flour an 8×8 inch baking pan.
In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy.
Add the eggs one at a time, beating well after each addition.
Stir in the mashed bananas and vanilla extract until well combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon.
Gradually add the dry ingredients to the banana mixture, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
Pour the batter into the prepared baking pan and spread evenly.
Bake in the preheated oven for 40 minutes, or until a toothpick inserted into the center comes out clean.
Remove from oven and allow the cake to cool in the pan for 10 minutes.
Turn the cake out onto a wire rack to cool completely before slicing into 8 equal pieces.