Preheat the oven to Gas Mark 4, 180˚C, fan 160˚C.
Grease a 1kg (2lb) loaf tin and line with nonstick baking paper.
Beat the butter and sugar with an electric beater for 5 minutes until creamy.
Mix in the bananas and then the eggs, one at a time. Sift in all the dry ingredients, add the chocolate, then fold them all together.
Tip the mixture into the tin and bake for 40-45 minutes, until springy and a skewer comes out clean. Allow to cool.
For the icing, sift the sugar into a bowl. Add the water slowly until the icing is thick enough to coat the back of a spoon. Drizzle over the cake and decorate with walnuts.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding icing. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
Add icing after the baked goods have fully defrosted and are at room temperature.