Break the chocolate into pieces and melt slowly in a heat-proof bowl set over a pan of just‑simmering water, making sure the base of the bowl doesn’t touch the water. Once melted, set aside to cool for 10 minutes.
Preheat the oven to 360F/180C/Gas 4. Grease and line two 20cm round cake tins with greaseproof paper.
Sift the flour, bicarbonate of soda and cocoa powder into a large bowl and make a well in the middle. Stir the oil, 75ml water and sugar into the melted chocolate, then whisk in the eggs. Pour half the mixture into the well and fold in, then mix in the remaining chocolate mixture.
Divide the mixture evenly between the tins and bake for 20‑25 minutes until a skewer inserted into the centre comes out clean. Remove the cakes from the tins and cool on a wire rack.
Make a chocolate ganache icing by melting the dark chocolate in a heat-proof bowl, as before. Stir in the milk then set aside to cool.
Whip the cream in a separate bowl until thick and moussey. Use a large metal spoon to fold in one large spoonful of the cream to the chocolate mixture. Continue to gently fold in the remaining cream until smooth.
Put one cake half on a cake stand and cover the top with jam. Place the other cake half on top. Spoon about half the ganache on top of the cake and slowly start to work it down the sides to cover the cake, adding spoonfuls of ganache to the sides until it is all covered. Put the remaining ganache on top and swirl into a pleasing pattern. Pile the fruit into the middle of the cake, slice and serve.