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Preheat the oven to fan 160C/ conventional180C/gas 4. Butter and line the base of two 20cm sandwich tins. Coarsely grate two thirds of the chocolate and chop the rest. Set aside.
Put the flour, baking powder and sugar into a large mixing bowl. Make a well in the centre and add the butter, eggs and milk (the butter must be very soft for this to work). Beat together with a hand held electric whisk for about 2 minutes until light and fluffy (or use a wooden spoon and beat well for 3 minutes). Lightly fold in the grated, chopped chocolate and mashed banana.
Divide the mixture evenly between the tins. Gently level the surface with the back of a metal spoon or spatula. Bake side by side on the middle shelf for 25-30 minutes until springy to the touch and beginning to shrink away from the sides.
Remove from the oven, leave to cool in the tins for 5 minutes. Turn out on a wire rack, remove the lining paper and leave to cool for about 1 hour before icing.
For the icing and filling, break up the white chocolate into a heatproof bowl and melt gently over a pan of hot, not quite simmering water until just melted. Remove from the pan and set aside for 5-6 minutes.
Cover a baking sheet with foil or waxed paper. Dip the cherries, one by one, into the chocolate to half coat them, then put them on the baking sheet to set.
Stir the crème fraîche into the remaining melted chocolate until smooth and shiny. Then let cool and thicken (5 minutes or so). Now transfer half of this mixture to another bowl. Pit another 200g/8oz cherries, cut them in half then add to one of the bowls with 2tbsp kirsch (optional) and stir until mixed. Put one sponge cake on a serving plate and spread the surface with the cherry mixture, then sandwich with the second sponge. Spread the rest of the chocolate mixture over the top and decorate with the dipped whole cherries. This cake is best eaten on the day it’s been made.