Preheat oven to 275 degrees F. Grease and flour a 9 x 13 pan, set aside.
In a small bowl, mix mashed banana with the lemon juice; set aside.
In a medium bowl, mix flour, baking soda and salt; set aside.
In a large bowl, cream butter and sugar until light and fluffy.
Beat in eggs, one at a time, then stir in vanilla.
Beat in the flour mixture alternately with the buttermilk. Then stir in banana mixture.
Pour batter into prepared pan and bake in preheated oven for one hour or until toothpick inserted in center comes out clean.
Remove from oven and place directly into the freezer for 45 minutes. This will make the cake very moist.
While the cake is cooling, mix together frosting ingredients until smooth. Add more milk if you need the frosting to be thinner or add more powdered sugar if you want to thicken the frosting up.
Spread frosting on cake and serve.