1. Grease two 9-inch layer cake pans; dust with flour; tap out any excess.
2. Sift flour, baking powder, baking soda, salt and cinnamon onto wax paper. Stir buttermilk into mashed bananas in a small bowl; reserve.
3. Beat butter or margarine, sugar, and eggs in the large bowl of electric mixer at high speed 3 minutes. Remove bowl from mixer.
4. 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.
5. Bake in moderate oven (350°) 30 minutes, or until centers spring back when lightly pressed with fingertip.
6. Cool layers in pans on wire racks 10 minutes; loosen around edge with a knife; turn out onto wire racks; cool completely.
7. Put layers together with rum butter cream; frost side and top with remaining frosting. Press remaining 1/2 cup nuts on sides.