Pre-heat oven to 275 F, and grease and flour (or spray), a 9x13 cake pan.
Mash banana and mix with lemon juice and set aside.
In a med bowl, mix flour, baking soda and salt. Set aside.
If you're making your own buttermilk, combine milk and vinegar (or lemon juice) in a small bowl, and set aside.
In a large bowl, cream butter and sugar.
Beat in eggs, one at a time. Stir in vanilla.
Beat in flour mixture alternately with buttermilk.
Stir in bananas.
Pour into cake pan and bake 1 hour and 10 mins (or until knife comes out clean). *if need be, cook longer, checking every 10 minutes until cake is fully cooked.
Immediately place in freezer for 45 minutes. (if not completely cool upon removal from freezer, let cool further before icing).
Cream butter and cream cheese.
Beat in vanilla and icing sugar on low until combined, then high until smooth.
Frost cake and enjoy!