This fabulously moist traybake will become a real family favourite. You can serve the cake without the final chocolate flourish, if you prefer.
Make the apple sauce. Put the apple in a medium pan with 220ml of the apple juice, and all of the sugar. Place over a high heat, bring to the boil, then reduce the heat to low and simmer for about 5 minutes, until the apples are pulpy. Leave them on the heat and mash them with a potato masher until smooth.
In a small jug, mix the cornflour and the remaining apple juice to a smooth paste. Add this to the pan with the apple. Cook for a further 5 minutes, then stir in the butter. Measure out 375g of the apple sauce and spread it out on a plate to cool (use the remainder as a dessert).
Make the sponge. Preheat the oven to 180°C/160°C fan/350°F/Gas 4, and put both types of raisin and the dates in the rum to soak.
Put the butter and both sugars in the bowl of a stand mixer fitted with the beater attachment and beat until light and creamy. Beat in the eggs one at a time, adding a little flour if the mixture curdles.
Sift together the flours, bicarbonate of soda, cinnamon, cocoa and salt into a bowl, then fold the dry ingredients into the creamed mixture along with the cooled apple sauce and the walnuts. Drain the raisins and dates from the rum (reserving the rum) and fold into the batter, then spread the batter into the prepared tin.
Bake the sponge for 25 minutes, or until just firm and a skewer inserted into the centre comes out clean. Remove from the oven and leave to cool slightly in the tin.
Meanwhile, make the ganache. Heat the cream in a small pan over a medium heat until hot but not boiling. Remove from the heat, add the chocolate, leave for 2 minutes, then stir until smooth. Allow to cool to a piping consistency.
Make the syrup. Bring the sugar, rum, reserved soaking rum and apple juice to the boil in a small pan over a high heat. Reduce the heat to medium and simmer for 5 minutes, until reduced by one third.
Prick the traybake all over with a skewer and pour the syrup evenly over the cake. Leave to cool for 5 minutes more, then turn out onto a wire rack to cool completely.
Make the candied nuts. Place the cream of tartar, sugar and golden syrup in a very small pan over a medium heat and dissolve the sugar. Continue heating for 3–5 minutes, until the mixture reaches 165°C on a sugar thermometer, or has turned a deep golden brown. Remove the pan from the heat and plunge it into cold water to stop the syrup cooking further. Add the walnut halves, stir to coat, then, using a fork, lift out each one and place on the sheet of foil to set.
Place the ganache in the piping bag fitted with the medium star nozzle. Cut the cake into 16 pieces. Pipe a stripe of ganache on top of each slice and finish with a walnut half.