Preheat the oven to 275°F. Grease and flour two round 9-by-2-inch cake pans. In a small bowl, mash the bananas and set aside. In a separate bowl, whisk together the flour, baking soda and salt. Set aside. In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and the sugar until light and fluffy, then beat in the eggs one at a time. Add the vanilla and then beat in the flour mixture alternately with the buttermilk. Fold in the mashed banana mixture and divide the batter evenly between the prepared cake pans. Bake the cakes for 55 to 60 minutes until a toothpick inserted comes out clean.Remove the cakes from the oven, place them on baking sheets to cool completely.