Preheat oven to 180°C conventional or 160°C fan-forced. Grease a 22cm round tin. Dust with cocoa.
Melt butter and chocolate carefully in a saucepan stirring over low heat constantly until smooth. Remove from heat and add sugar while the chocolate mixture is still hot.
Whisk in vanilla, buttermilk, eggs, then sifted in the dry ingredients and gently whisk until smooth. Pour mixture into prepared tin.
Bake for 50 – 55 minutes or until skewer comes out clean. Tip: If cake is cooked but slightly sticky in the centre, remove cake from oven. Cover cake with tea towel for 10 minutes. Remove tea towel, allow cake to further cool in tin. Turn cake onto wire rack to completely cool, before icing.
To make the icing sift icing sugar and cocoa powder into a bowl and add margarine spread or softened butter and milk. Mix with a wooden spoon until smooth. Add a few extra drops of milk if required. Spread over cooled cake.