Sift together and set aside in large bowl the flour and sugar.
In saucepan over medium heat bring to a boil; 125g butter, 125g margarine, 1 cup (250ml) water and 4 tablespoons cocoa.
Pour cocoa mixture over flour and sugar mixture. Stir in buttermilk, eggs, bicarb soda and 1/2 teaspoon vanilla. Mix well and pour into a greased and floured sheet pan.
Bake in a preheated 200 degrees C oven for 20 minutes.
To Make icing: Five minutes before cake is done, bring to boil in a saucepan 125g butter, 6 tablespoons milk and 4 tablespoons cocoa. Remove from heat and immediately stir in the icing sugar and 1/2 teaspoon vanilla essence. Beat until smooth and immediately pour icing over cake. Cake may be served warm or at room temperature.