Preheat oven to 350 degrees. Grease & flour two 8-or 9-inch round cake pans, and line the bottoms with parchment.
Cream the butter in large bowl with electric mixer on medium speed. Gradually add the sugar, 1/4 cup at a time, beating until pale & fluffy. Blend in vanilla. Add eggs one at a time, beating well after each addition.
In another bowl,sift the flour with the baking powder,soda & salt.
In a another small bowl, combine mashed bananas & buttermilk.
To the creamed mixture add in the following order, beating on low speed after each addition just until blended: A third of the flour mixture,half the banana mixture,half the remaining flour mixture,the remaining banana mixture,then the remaining flour mixture.
Spoon batter into pans. Bake 25-30 minutes, until toothpick inserted in center comes out clean. Let cool 5 to 10 minutes before inverting onto wire racks. Cool before frosting.