1. Preheat oven to 160°C. Grease and line the bases of 2 x 20cm sponge sandwich tins with baking paper.
2. Place the cake mix, soft butter or margarine spread, eggs and milk into a medium mixing bowl.
3. Using an electric mixer beat on low speed for 30 seconds to combine. Increase speed to medium and mix for 2 minutes, scraping down sides of bowl occasionally.
4. Divide and pour mixture evenly into the tins.
5. Bake for 20 to 25 minutes or until the cakes spring back when lightly touched. Leave in the tins for 10 minutes before turning out onto a wire rack to cool.
6. To make icing beat all ingredients together until smooth. Add more water if necessary to form a spreadable icing.
7. When cold, sandwich together with one quarter of the icing, then spread remaining icing over the top and sides of the cake.
8. Press chopped walnuts onto the sides of the cake. Decorate with walnut halves