Preheat oven to 275F. Grease (or spray with oil) and flour a 9 x 13 pan.
PREPARE BANANAS: In a small bowl, mash bananas, mix with 2 teaspoons lemon juice and set aside.
PREPARE BUTTERMILK: Measure out the buttermilk (or milk plus lemon) in a measuring cup. Set aside.
PREPARE DRY INGREDIENTS: In another bowl, stir together flour, baking soda and salt. Set aside.
MAKE BATTER: In a large bowl, cream butter and sugar until fluffy with an electric mixer. Beat in eggs and vanilla. Alternate adding flour mixture and buttermilk a bit at a time - and beat until smooth. Stir in banana mixture. Pour batter into prepared pan.
BAKE AND FREEZE: Bake the cake for an hour or just until a toothpick comes out clean. As ovens vary, start checking after 55 minutes. Remove cake from oven and place it directly in the freezer, uncovered, on a towel for 45 minutes. This apparently makes the cake very moist - and it does!
MAKE ICING: Using a food processor: Add all icing ingredients into the processor and process until smooth, about 2 minutes. If too dry, add a tablespoon more milk or, if too thin, add a bit more icing sugar - until you get the consistency you want. (Alternatively, use an electric mixer, beating butter first, adding icing sugar and cocoa next, then vanilla and milk.) When the cake is fully cooled, ice the cake.