Preheat the oven to 180C/350F/Gas 4. Grease and line a square cake and a round cake tin.
For the cake, place all of the cake ingredients, except the boiling water, into a large mixing bowl. Using a wooden spoon, or electric 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.
Prepare a cardboard base for the cake. It should measure about 15 by 15 inches. Cover with aluminium foil or paper doilies as desired.
Place the square layer on the platter so that one corner faces down at the bottom of the platter, forming a diamond shape.
Cut the round layer in half, then place the cut side of each half of the round layer against one upper side of the square layer to form a one-layer heart.
For the chocolate icing, heat the 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 hours, or until thick enough to spread over the cake.