Preheat oven to 180C.
Cream butter and sugar together with mixer. Add eggs. Stir in mashed bananas.
Sift flour and Bicarb, then stir in to butter mixture. Combine milk (or water) and sour cream and stir in, in 2 batches.
Pour into a deep 23 cm pan. Bake in moderate oven (180C) for about 1 hour.
Beat together first 4 ingredients, then add icing sugar beating until smooth. Let cake cool before icing.