Preheat the oven to 275 degrees. Grease a 13 x 9 inch baking pan.
In a small bowl, mix the mashed bananas and lemon juice.
Beat the butter with a mixer at medium speed until creamy. Gradually add the sugar, beating well. Add the vanilla and eggs, 1 at a time, beating well after each addition.
Combine the flour, baking soda, and salt in a bowl. Add the flour mixture to the butter mixture, alternately with the buttermilk. Beat at low speed until blended after each addition, scraping down the bowl as needed. Stir in the banana mixture. Pour the batter into the prepared pan.
Bake 1 hour or until a wooden pick inserted in the center comes our clean. Cool Completely in pan on wire rack (or freeze 45 minutes for faster cooling).
Cream Cheese Frosting
Beat the butter and cream cheese with a mixer at medium-high speed until creamy, 3 minutes. Gradually add the powdered sugar at low speed until blended. Increase the speed to medium. Add the vanilla and salt. Beat 1 minute until light and fluffy. Spread the frosting on the top of the cake.