Grease two 9 x 1 1/2-inch layer-cake pans; dust lightly with flour; tap out any excess. Sift flour, baking powder, baking soda, salt, and cinnamon onto wax paper; reserve. Stir buttermilk into mashed bananas in a small bowl, reserve.

Beat butter or margarine, sugar and eggs in large bowl of mixer at high speed 3 minutes. Remove bowl from mixer. Stir in flour mixture alternately with banana-milk mixture, beating after each addition until batter is smooth. Stir in vanilla and 1/4 cup chopped nuts; pour batter into prepared pans. Bake in moderate oven (350°F) 30 minutes, or until centers spring back when lightly pressed with fingertip.

Cool layers in pans on wire racks 10 minutes; loosen around edges with a knife; turn out onto wire racks; cool completely. Put layers together with rum butter cream frosting; frost side and top with remaining frosting.

Press remaining 1/2 cup chopped nuts on sides of cake. Dip banana slices in orange or pineapple juice to keep white; garnish top of cake, if you wish.