- Preheat oven to 275°F (135C).
- 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.