Step 1: Heat oven to gas mark 6/200°C (180°C in a fan oven).
Step 2: Put sugar and eggs into bowl of an electric mixer and whisk for 10 mins, until pale and thick enough to leave a trail when whisk is lifted.
Step 3: Sift flour and cocoa into mixture and fold in carefully using a large metal spoon.
Step 4: Pour mixture into prepared tin and use a spatula to smooth evenly into corners. Bake in the centre of oven for 10-12 mins, until risen and just firm to the touch. Leave to cool in the tin. Sprinkle with kirsch, if liked.
Step 5: Meanwhile, to make chocolate ganache, heat whipping cream to a gentle simmer, then pour it over chocolate and stir until smooth. Leave to cool until thickened.
Step 6: Drain juice from can of cherries into saucepan, add jam and boil until reduced to 4tbsp. Add cherries and stir to coat.
Step 7: Turn cake out of tin, peel off baking paper, then trim off all edges. Cut in half horizontally to give 2 long slices of cake.
Step 8: Spread ganache over top of both cakes. Whip double cream until it just holds its shape. Spread two thirds of cream over one slice and top with drained cherries.
Step 9: Arrange remaining cream in heaped teaspoonfuls down the centre of other slice of cake and top with fresh cherries. Place this slice on top of the other, sprinkle with grated chocolate or chocolate curls and chill until ready to serve.