Preheat the oven to 180C. Lightly oil and line a 9 inch (23cm) round cake tin.
In a large bowl cream together the butter, vanilla essence and sugar until light and fluffy.
Beat in the eggs yolks - one at the time.
In another bowl sift together the flour and cocoa powder and gently fold into the egg mixture together with the buttermilk.
In another bowl whisk the egg whites until soft peaks form and then fold carefully into the chocolate mixture in two batches.
Spoon the mixture into the prepared cake tin and bake in the preheated oven for 40-50 minutes or until firm. Make sure that you check with a wooden skewer to make sure the cake is done.
Cool slightly in the cake tin and then turn out carefully onto a wire rack and leave to cool down completely.
Whilst the cake is cooling down, melt the chocolate and butter. You can do this either very carefully in the microwave in 20 sec bursts and mixing after each 20 seconds.
Alternatively, you can place the chocolate and butter in a heatproof bowl over a simmering pan of water. Stir the chocolate mixture and then leave to cool down at room temperature until slightly thicker and easy to spread.
To cover your chocolate cake, split the cake first horizontally in half and use the chocolate mixture to sandwich the two cakes together. Spread a little on the top too and on the sides of the cake to keep the cake nice and moist.
To make the cake extra special you can also whip some double cream and decorate the top of the cake and add sprinkles, chocolate pieces or other chocolate decorations. Either way, even without the cream, this is very delicious cake!