Preheat gas or electric oven to 180°C (350°F).
Grease and flour cake pan or lightly grease non-stick cake pan.
In a large mixing bowl, place cake mix, 2 eggs, 3 tablespoons soft polyunsaturated margarine and 2/3 cup milk.
Using an electric mixer, mix on low speed until ingredients are combined. Mix on medium speed for 2 minutes. Scrape down sides of bowl occasionally.
Pour batter into prepared cake pan.
Bake for 40 - 45 minutes. Cake is baked if it springs back when lightly touched in the centre.
Invert cake onto a cooling wire.
Note: Allow cake to cool completely before icing.
To Prepare Icing
Empty contents of icing sachet into a small mixing bowl with 3 tablespoons soft polyunsaturated margarine or soft butter. Mix with wooden spoon until smooth. Spread icing over top of cake.