1Set oven to 275. Lighlty spray a 9" x 13" baking pan w/ cooking spray. In medium bowl, mash bananas and add lemon juice. Mix, set aside.
2In another medium bowl, mix flour and baking soda. In lg bowl, cream butter and sugar until fluffy. Beat in eggs, one at a time.beat in vanilla. Beat in flour mix, alternately w/ buttermilk until smooth. Pour into baking pan. Bake about 1 hr. My oven actually took 45 mins. Check w/ an inserted toothpick in center of cake. If it comes out clean, it's done.
3Let the cake cool 5 mins, then put into freezer, ( this helps keep it moist.) Let cake sit in freezer for 15 mins. In medium bowl, spoon frosting into bowl. Add remaining mashed banana. Beat until mostly smooth. Spread on cake. Sprinkle w/ chopped walnuts.