Preheat the oven to 180ºC (350ºF, gas mark 4). Grease and line an 18cm (7in) round cake tin.
Reserve half a banana for the icing and place the rest of the bananas in a mixing bowl. Mash until smooth, then set aside.
In a large mixing bowl, beat together the butter and sugar until pale and fluffy. Gradually add the eggs and bananas, and beat until smooth. Stir in the flour, salt and bicarbonate of soda and mix thoroughly. Quickly mix in the walnuts, then pour into the baking tin.
Bake in the oven for 60-75 mins or until a skewer inserted into the middle comes out clean. Turn out the banana cake and leave to cool on a wire rack.
To make the icing, place the butter and cocoa powder in a small saucepan. Heat until melted. Mash the reserved banana until very smooth. Add the melted butter mixture to the mashed banana and beat well, then add the icing sugar and vanilla extract. Beat thoroughly until very smooth. Use to ice the top of the banana cake once it has cooled. Decorate with chopped walnuts if you wish.