Preheat oven to 275F/135C. Grease (or spray with oil) and flour a 9 x 13 inch pan (22 x 33 cm).
PREPARE BANANAS: In a small bowl, mash bananas with a fork, mix with 2 teaspoons lemon juice and set aside.
PREPARE BUTTERMILK: Measure out the buttermilk (or milk plus lemon) in a medium bowl or measuring cup. Set aside.
PREPARE DRY INGREDIENTS: In a separate 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 1 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 tea towel for 45 minutes. This apparently makes the cake very moist - and it works!
MAKE FROSTING: Using a food processor: Add all buttercream/frosting ingredients into the processor and process until smooth, about 2 minutes. If too dry, add a small amount of extra milk (1 tablespoon) or, if too thin, add a bit more icing sugar - until you get the consistency you want. (Alternatively, use an electric or hand mixer and a mixing bowl - beating butter first, adding icing sugar and cocoa next, then vanilla and milk.) When the cake is fully cooled, frost the cake.