Preheat oven to 275 degrees and grease or spray a 9x13 cake pan.
Mash banana in a small bowl and mix with the juice of one lemon and set aside.
In a medium bowl, mix flour, baking soda, and salt. Set aside.
If you're making your own buttermilk, mix together the milk and lemon juice and set aside.
In a large bowl (or a large stand mixer), cream the butter and sugar.
Beat in the eggs, one at a time. Stir in the vanilla extract.
Beat in the flour mixture alternately with buttermilk.
Stir in bananas.
Pour into cake pan and bake for 1 hr and 10 minutes (or until knife comes out clean). After an hour, check every 5 to ten minutes.
Immediately place in the freezer for minutes. Make sure it's completely cool before frosting.