Preheat oven to 180 degrees/Gas mark 4. Grease and line 2 20cm sandwich tins.
For the cake, Place all of the cake ingredients except the boiling water, plain chocolate and double cream into a large mixing bowl. Using a wooden spoon, or eletric whisk, beat the mixture until smooth and well combined.
Add the boiling water to the mixture, a little at a time, until smooth (The cake mixture will now be very liquid).
Divide the cake batter between the sandwich tins and bake in the oven for 25-35 minutes, or until the top is firm to the touch and a skewer inserted into the centre of the cake comes out clean
Remove the cakes from the oven and allow to cool completely, still in their tins before icing.
For the chocolate icing, heat the plain chocolate and cream in a saucepan over a low heat until the chocolate melts. Remove the pan from the heat and whisk the mixture until smooth, glossy and thickened. Set aside to cool for 1-2 hour, or until thick enough to spread over the cake.
To assemble the cake, run a round-bladed knife around the inside of the cake tins to loosen the cakes. Carefully remove the cakes from the tins.
Spread a little chocolate icing over the top of one of the cakes,then carefully top with the other cake.
Transfer the cake to a serving plate and ice the cake all over with the chocolate icing, using a palette knife.