Pre-heat oven (190 °C, Fan 170 °C, 375 °F, Gas 5), and line a 20x20cm (8x8 inch) square baking tin with baking paper.
In a large bowl, stir together all dry ingredients until thoroughly mixed up, then mix in all wet ingredients until a thick batter forms. Scoop in to the lined cake tin and spread out level.
Bake in the centre of the oven for 25-30 minutes. Towards the end of this time test by inserting a cocktail stick into the middle of the cake. It should come out clean. If there is wet batter on the stick, return cake to the oven and cook for a few minutes more and test again. Repeat as needed until the stick comes out clean. When the cake is done, allow it to cool completely on a wire rack before applying icing.
To make the icing, cream together the margarine and icing sugar. A flexible spatula works great for this, but you can use a spoon. Spread on to the cake, then decorate with walnut pieces.
Notes
Golden icing sugar is a delicious option for this cake, but regular white sugar works great too if golden can't be found.Storage: Cover with an air tight wrapper or container. Keeps well for up to three days in a cool place.