Preheat the oven to 275. Grease and flour a 9x13 metal baking pan.
Mix and mash the bananas with lemon juice, and reserve for later.
In a medium bowl, mix the flour, baking soda and salt, and set aside.
In a large bowl, cream together the butter and sugar. Beat in the eggs, one at a time, and stir in the vanilla.
Alternating between the two, gradually beat the flour mixture and the buttermilk into the creamed butter-egg-sugar mixture.
Stir in the mashed bananas until just mixed.
Pour batter into the prepared baking dish. Bake for 1 hour 10 minutes, until an inserted knife comes out clean.
While the cake bakes, place a sheet pan in the freezer.
After baking, remove the sheet pan from the freezer and line with a towel. Remove the baked cake from the oven and place on the towel-lined frozen sheet pan. Place in the freezer for 45 minutes to 1 hour, until completely cooled.
While the cake cools, prepare the frosting. Cream the butter and cream cheese together. On the low setting, beat in the vanilla extract and the powdered sugar until combined. Mix on high until smooth.
Frost the cooled cake and serve.