With electric mixer, cream brown sugar and butter. Spread mixture over bottom of a 9 inch square baking dish with 2 inch high sides. Arrange sliced bananas on top of mixture, covering completely. Set aside.
Combine flour, baking soda, baking powder and salt in a small bowl.
Mix mashed bananas, buttermilk and vanilla in another small bowl.
Using an electric mixer, cream 1 1/4 cups sugar and shortening until fluffy. Add eggs 1 at a time, beating well after each adition. Add dry ingredients alternately with buttermilk mixture, mixing until just combined.
Pour batter over bananas in pan. Bake until cake pulls away from sides of pan and a pick inserted into the center of cake comes out clean, about 1 hour and 5 minutes. Cool slightly.