Preheat oven to 275 degrees.
Grease and flour a 9 x 13 pan.
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 3/4 cup butter and 2 1/8 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 for one hour or until toothpick inserted in center comes out clean.
Remove from oven and place directly into the freezer for 45 minutes. This will make the cake very moist.
For the frosting, cream the butter and cream cheese until smooth.
Beat in 1 teaspoon vanilla.
Add icing sugar and beat on low speed until combined, then on high speed until frosting is smooth.
Spread on cooled cake.
Sprinkle chopped walnuts over top of the frosting, if desired.