In a small bowl, mix mashed banana with the lemon juice; set aside.
In a medium bowl, mix flour, baking soda and salt; set aside.
In a large bowl, cream ¾ cup butter and 2 cups sugar until light and fluffy.
Beat in eggs, one at a time, then stir in 2 tsp vanilla.
Beat in the flour mixture alternately with the buttermilk.
Stir in banana mixture.
Pour batter into prepared pan and bake in preheated oven a until toothpick inserted in the center comes out clean. I used a glass 9x12 pan and baked for 55 minutes. Your baking time could vary. Check for doneness at 50 minutes.
Remove from oven and place directly into the freezer for 45 minutes or until cooled. This will make the cake very moist.
For the frosting, cream the butter and cream cheese until smooth.
Beat in 1 teaspoon vanilla.
Add confectioners sugar and beat on low-speed until combined, then on high-speed until frosting is smooth.