175g butter, completely softened, plus extra for lining the tin
175g golden caster sugar
1 tsp baking powder
1-2 tbsp milk
100g 70% dark chocolate, melted and cooled
500ml double cream
500g milk chocolate, chopped
Make the icing first (it takes a while to cool and thicken). Put the chocolate into a bowl. Pour the cream into a pan and heat until simmering. Take off the heat and pour it over the chocolate. Stir until the mixture is smooth. Cool, then chill, until really thick but spreadable.
Heat the oven to 180C/fan 160C/gas 4. Butter and base-line 2 x 18cm sandwich tins with baking parchment. Take 3 tbsp flour from the full flour amount and put them back in the flour bag - the cocoa powder will make up the difference.
Put all the cake ingredients except the chocolate in a large bowl. Beat them together with an electric whisk (or whizz in a food processor) until you have a creamy mixture, then fold in the melted chocolate. Add a little more milk if the mix is too stiff - it should fall easily from a spoon.
Divide the mixture between the 2 tins and level. Bake on the same shelf in the oven for 20-25 minutes or until the sponge springs back when pressed. Cool for 5 minutes, turn out onto a wire rack, peel off the paper and cool completely. Cut each in half horizontally so that you have 4 layers altogether.
Sandwich the layers together with a little icing, then spread the rest on top thickly.