Preheat the oven to 190°C /375°F/Gas Mark 5. Beat together the cooking oil or butter, sugar, eggs and vanilla extract until mixture is smooth; about half a minute in the food processor should do it.
Pour in the melted chocolate and mix together thoroughly. Sieve the plain and self-raising flour, drinking chocolate and baking powder together into a large mixing bowl. Make a well in the centre, add the runny chocolate mixture and fold everything together. Gently stir in the finely chopped beetroot.
Divide the mixture among two greased, lined 18cm cake tins. Bake in the oven for 30-40 minutes until cooked through. Cool on wire rack and remove the greaseproof paper from the base of each cake.
Spread one cake with jam and the other with cream and sandwich the two together. To make the ganache, place the cream in a saucepan and bring to the boil, stirring continuously. Add the chocolate and stir until it has melted and the mixture is combined. Pour into a bowl and whisk until fluffy and cool. Spread the ganache smoothly and evenly over the top of the cake.