Lightly oil a 20 x 25cm springform cake tin, line with cling film, then lightly oil again. Set aside.
Put 200g chocolate in a heatproof bowl and melt in the microwave in 20-second bursts, stirring until smooth. Alternatively, melt the chocolate in a heatproof bowl set over a pan of simmering water, making sure the bowl doesn’t touch the water. Leave to cool for 10 mins. Whisk together the double cream, soured cream, icing sugar and vanilla bean paste until soft but spoonable (it should be quite loose at this stage but will firm up in the fridge later). Transfer half the mixture to a second bowl, then fold in the melted chocolate.
Tip half the cherries and all the kirsch into a saucepan. Chop the remaining cherries and set aside. Line the bottom of the prepared tin with a layer of bourbon biscuits, then spoon over half the chocolate cream, half the chopped cherries, and half the vanilla cream. Repeat with a second layer of biscuits and the remaining chocolate cream, chopped cherries and vanilla cream. Cover and chill overnight.
Tip the caster sugar into the saucepan with the remaining whole cherries and kirsch. Cook over a low heat for about 5 mins, stirring until the sugar has dissolved. Turn up the heat, bring to the boil and continue cooking on a high heat for 8-10 mins, or until the liquid has reduced to a syrupy consistency that coats the back of a spoon. Leave to cool completely.
Carefully remove the cake from the tin, and peel away the cling film. Leave at room temperature for 30 mins to soften slightly. Roughly chop the remaining dark chocolate. Spoon the cooled cherries and syrup over the cake, and scatter over the chocolate. Cut into 12 squares to serve. Will keep, covered in the fridge, for up to three days.