Preheat the oven to 180 °C.
Grease two round 20 cm cake tins with butter or non-stick spray and line the bases with baking paper.
Sift the dry ingredients together.
Cream the butter and castor sugar in a separate bowl until light and fluffy.
Add the eggs one by one, beating well after each addition. Stir in the vanilla essence.
Add the dry ingredients but do not stir.
Pour the boiling water on top and beat until the mixture is smooth.
Divide the batter between the two cake tins and bake for 25 to 35 minutes or until the cake shrinks slightly from the sides of the tins.
Turn out onto a wire rack and leave to cool.
In a saucepan heat the cream to just below boiling point.
Add the chocolate, stirring until melted.
Remove from the heat and leave to cool and thicken.
Halve each layer of cake horizontally and spread with a generous layer of icing.
Sandwich the four layers together and cover the top and sides with the remaining icing.