Preheat oven to 180°C. Grease a 15-cm x 25-cm slice tin. (I like to line tin with baking paper)
Sift flour and spices into a bowl. In a separate bowl, cream butter and sugar, then add the mashed banana and beat for 2 minutes. Add the egg and beat well. Mix in the milk and flour mixture alternately. Stir in the walnuts. Put into prepared tin and bake for 25-30 minutes.
To make the icing, combine the butter with the water and lemon juice and add sufficient sifted icing sugar to achieve the desired consistency.
While the slice is still warm, spread with lemon icing and cut into squares.
RECIPE NOTES
I found it easier to cut the slice into squares once it had cooled.