Cream butter and sugar together. Add one egg at a time, beating till smooth. Blend in mashed bananas. In second bowl, stir flour with baking soda, baking powder, salt and nuts. Add to banana mixture stirring only to moisten. Pour into a greased/floured loaf pan. Bake at 350 degrees for about 1 hour until inserted toothpick comes out clean. Let stand 10 minutes. Remove from pan and place on wire rack to cool.