For the cake:
1) Preheat the oven to 180C/Gas 4. Line a 23-cm baking tray with heavy-duty aluminium foil.
2) In a large bowl, combine the butter and sugar. Beat at medium speed with an electric mixer until fluffy. Add the eggs one at a time, beating well after each addition. Beat in the vanilla essence.
3) In a small bowl, combine the flour, cocoa, baking powder and salt. Gradually add to the butter mixture, beating until combined. Spoon the batter into the prepared pan.
4) Bake for 22 minutes or until a wooden pick inserted in the centre comes out clean. Spread the chocolate icing over the hot cake. Let it cool completely. Cut into squares and serve with whipped cream, if desired.
For the chocolate icing:
1) In a medium saucepan, melt the butter over medium-low heat. Add the cocoa, buttermilk and 125g of icing sugar. Stir in the chocolate bars and allow to melt. Stir in the remaining icing sugar and immediately spread over the hot cake.