Preheat oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan or line with parchment paper.
In a medium bowl, mash the bananas until smooth.
In another bowl, whisk together the all-purpose flour, baking soda, and salt.
In a large mixing bowl, cream the butter and sugar until light and fluffy (2-3 minutes).
Beat in the eggs one at a time, followed by the vanilla extract.
Mix in the mashed bananas until combined.
Gradually add the dry ingredients, 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 for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.