Cream together butter and sugar until light and fluffy. Thoroughly mix in the egg and mashed bananas. Add flour, baking powder, baking soda, salt and chopped nuts, but do not over-mix.
Line base of a large loaf tin or 21cm cake tin with baking paper, pour in mixture, and bake at 180°C for 50 mins. After cooling, turn out onto wire rack.
Cream Cheese Icing:
Mix together butter and cream cheese until creamy. Beat in the Chelsea Icing Sugar. Spread on top of the cake and sprinkle with walnuts.
* If using hazelnuts, toast at 180°C for 5 to 10 mins until golden brown. Rub skins off using paper towel and set aside.