Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x13 inch pan.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Combine the flour, baking soda and salt; stir into the creamed mixture alternately with the buttermilk. Fold in the mashed bananas.
Pour batter into prepared pan. Bake for 30 to 40 minutes in preheated oven. Cake is done when it springs back to the touch.