Preheat oven to 275 degrees F (135 degrees C). Grease and flour a 9x13 inch pan. In a small bowl, mix mashed bananas with lemon juice, set aside. In a medium bowl, mix flour, baking soda and salt. Set aside.
In a large bowl, cream 3/4 cup butter and 2 1/8 cups sugar until light and fluffy. Beat in the eggs one at a time, then stir in 2 teaspoons vanilla. Beat in the flour mixture alternately with the buttermilk. Stir in banana mixture. Pour batter into prepared pan.
Bake in preheated oven for 1 hour, or until a toothpick inserted into the center of the cake comes out clean. Remove from oven and place directly into freezer for 45 minutes. This will make the cake very moist.
For the frosting: In a large bowl, cream 1/2 cup butter and cream cheese until smooth. Beat in 1 teaspoon vanilla. Add confectioners sugar and beat on low speed until combined, then on high until frosting is smooth. Spread on cooled cake.