2 x boxed devil’s food cake mix, made as per the box ingredients
200ml double cream
200g dark chocolate, chopped
200g caramel sauce
Make the cream filling: line a 20cm cake tin with cling film. Soak the gelatine sheets in water for 5 minutes. Heat 150ml of the cream in a small saucepan to hot but not boiling. Squeeze the water out of the gelatine sheets and add them to the cream. Keep over a very low heat while you dissolve the gelatine sheets, stirring constantly, and take off the heat as soon as they are dissolved. Put to one side to cool completely, around 30 minutes.
Whip the remaining 550 millilitres of cream to soft peaks. Gradually add in the icing sugar, then fold through the cooled gelatine mixture. Spoon it into the lined cake tin and chill for 3 hours until fully set.
Make the cakes: preheat the oven to 180 C (160 C fan). Grease and line two 20cm springform cake tins.
Mix together the 2 devil’s food cake mixtures as per the instructions on the box. Bake for 35-40 minutes until a skewer comes out clean. Let cool in the tins for 30 minutes, then take out of the tins and leave to cool fully on a wire rack. Trim the tops so they are completely flat, and turn one upside down - this will be the top of your cake.
Make the caramel ganache: warm the cream in a saucepan until hot but not boiling. Switch off the heat and add in the chopped chocolate, whisking constantly until it’s fully mixed together. Pour in the caramel sauce and carry on whisking until it’s thick and glossy. Put to one side to cool slightly, around 15 minutes.
Assemble the cake: put the first cake onto a wire rack or decorating turntable (save the upside-down cake for the top, so that you get a very square edge). Carefully turn out the set cream filling on top of that, and remove the clingfilm. Position the second cake on top, still upside-down, then use a hot knife to trim the cream filling so that you have a straight edge all around the cake. Pour the caramel ganache over the top of the cake, using a palette knife to smooth it down the edges of the cake until the whole thing is covered.
Put it into the fridge for at least one hour, to help the ganache set, then carefully transfer it to a serving plate. For best results, use a hot knife to slice cleanly through the ganache and cream filling.