Preheat oven to 180 degrees C. Grease and flour a 20x30cm cake tin.
Cake: In a medium bowl, combine the buttermilk and bicarb soda. Set aside in a warm place.
In a large bowl, mix together sugar, eggs, cocoa powder, oil and vanilla until blended. Stir in the boiling water then gradually beat in the flour. Stir in the buttermilk mixture.
Pour batter into prepared cake tin. Bake in the preheated oven for 30 minutes or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.
Sweet Fudge Icing: In a saucepan, combine sugar, cocoa powder, milk, butter and vanilla essence. Bring to a boil and cook for 1 minute. Remove from heat. Using an electric mixer, beat icing for 3 minutes or until it thickens to spreading consistency. Pour over completely cooled cake.