Preheat oven to 160c/325f / Gas 3. Grease two 20.5 (8in) sandwich tins and set aside.
Sift all dry ingredients together, then add all the remaining ingredients and whisk until smooth.
Divide the mixture between two sandwich tins and bake for 20-25 mins. Rest in tins for 10mins before turning out onto a wire rack to cool completely.
For the topping, place a saucepan of hot water, then turn down to a low heat and slowly melt the chocolate and 75ml (3fl oz) of cream and butter and stir until completely melted. Set aside to cool.
Whip the rest of the cream until stiff and use to sandwich cakes together. Spread the cooled chocolate cream thickly over the top.
Top tip
Check if the cake is cooked, if you gently press in the middle it should spring back quickly.