Preheat oven to 350. Grease (or spray with oil) and flour a 9 x 13 inch pan.
Prepare the bananas: In a small bowl, mash bananas with a fork, mix with 2 teaspoons lemon juice and set aside.
Prepare the dry ingredients: In a separate bowl, stir together flour, baking soda and salt. Set aside.
Make the 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 refrigerate: Bake the cake for 45-50 minutes or just until a toothpick comes out clean. As ovens vary, start checking after 45 minutes. Remove cake from oven and place it directly in the fridge, uncovered, for 45 minutes. This apparently makes the cake very moist – and it works!
Make the icing: Using a hand mixer, add the softened butter to a medium size bowl and and beat the butter for 1 minute. Add in the powdered sugar, cocoa, vanilla, and milk and beat 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 powdered sugar – until you get the consistency you want. Beat until fluffy, thick and super silky.
Frost the cake: Use the back of a spoon to make a wavey effect of buttercream on top of the cooled banana cake.