Heat the oven to 275°F (135°C). Grease and flour a 9 x 13-inch baking pan, coating the corners and sides carefully so the chilled cake releases cleanly.

Mash the very ripe bananas until mostly smooth, then stir in the lemon juice. Set the banana mixture aside while you prepare the batter.

Whisk together the flour, baking soda, and salt in a medium bowl. In a separate large bowl, cream the softened butter and granulated sugar until light and fluffy.

Beat the eggs into the butter mixture one at a time, mixing well after each addition. Stir in the vanilla extract until the mixture is evenly blended.

Add the flour mixture alternately with the buttermilk, beginning and ending with the dry ingredients. Mix only until combined, then gently fold in the banana mixture.

Spread the batter evenly in the prepared pan. Bake for approximately 1 hour, checking the center with a toothpick; it should come out clean or with a few moist crumbs, and some ovens may require 10 to 15 additional minutes.

Place the hot cake directly in the freezer for about 45 minutes. Keep the pan level and uncovered so condensation does not collect on the surface.

For the frosting, beat the softened butter and cream cheese until smooth. Add the vanilla, then gradually beat in the confectioners' sugar until creamy.

Frost the thoroughly chilled cake, slice it into 20 to 24 servings, and serve. Refrigerate leftovers in a covered container.