100g dark chocolate, chopped plus extra to decorate
Heat the oven to 180C/fan 160C/gas 4. Butter and base-line 2 x 18cm sandwich tins. Take 3 tbsp flour from the full flour amount and put it back in the bag - the cocoa powder will make up the difference.
Put all the cake ingredients (not the double cream) in a large bowl and beat them together with an electric whisk until you have a creamy mixture. Add a little milk if the mix is too stiff - it should fall easily from a spoon. Divide the mixture between the prepared tins and level the surfaces of the cake mixtures. Bake on the same shelf in the oven for 20-25 minutes or until the sponge springs back when pressed. Cool for 5 minutes, then turn out onto a wire rack, peel off the base paper and cool completely.
To make the icing, melt the butter and chocolate together in a microwave, or in a bowl set over a pan of simmering water. Stir well, then leave until it’s cool enough to spread easily and will run a little.
Whip the cream and use it to sandwich together the cakes. Spread chocolate ganache over the top of the cake, letting some run over the edge. Decorate with chocolate curls.