Preheat oven to 190C/380F/Gas 5.
Butter two 20cm/8in round baking tins then line with buttered baking parchment.
Beat the butter and sugar together until they are light and fluffy.
Sieve the flour, baking powder and cocoa together into a clean bowl.
Beat the eggs into the butter and sugar mixture, one at a time.
Gently fold in the sieved flour with a metal spoon.
Divide the mixture between the two prepared cake tins and place into the oven to bake for 20-25 minutes, or until the cake tops spring back when lightly pressed with your finger.
Remove from the oven and leave the cakes to cool in their tins for five minutes, then turn out onto a wire rack, remove the baking parchment and leave until they are completely cool.
Meanwhile, for the filling, drain the cherries, reserving six tablespoons of the syrup. Place the reserved syrup into a small pan and heat gently.
Mix the arrowroot with two tablespoons of water and add to the heated syrup, stirring constantly until the sauce has thickened.
Add one tablespoon of the rum, then remove from the heat and leave to cool.
Slice the sponge cakes in half and sprinkle each half with the remaining rum.
Cut the drained morello cherries in half, (removing their stones if necessary). Leave 10-12 cherries whole for decoration.
Whip the cream and the sugar together in a bowl until soft peaks form when the whisk is removed.
Reserve half of the cream mixture to cover the top of the cake. Stir the halved cherries and the heated syrup.to the remaining half of the cream.
Spread a third of the cream and cherry mixture onto one side of one layer of the cake. Add a second layer and spread with more of the cherry cream. Add a third layer of cake and spread the remaining cherry cream over the top.
Top with the final piece of cake and top with the reserved whipped cream.
Grate the dark chocolate over the top and decorate with the reserved whole cherries.
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