Preheat oven to 300 degrees F. Grease and flour 9 x 13 pan. In small bowl mix mashed bananas with lemon juice and set aside. In medium bowl, mix flour, baking soda and salt. With electric mixer beat butter and sugar in separate bowl until light and fluffy. Beat in eggs, one at a time, then stir in vanilla. Beat in flour mixture and alternate with buttermilk. Stir in banana mixture.
Bake 50 minutes to 1 hour or until toothpick comes out clean (don't overbake). Remove from oven and place directly in freezer for 45 minutes...no lid or cover. When cake is cool, frost with cream cheese.
For frosting, beat butter and cream cheese until smooth with electric mixer. Beat in vanilla. Gradually add powdered sugar and beat on low speed until combined, then on high until frosting is smooth. If frosting is too stiff beat in milk.