Heat oven to 180°C. Grease and line the base of 3 x 23cm cake tins. Boil the kettle. Put the butter and 100g chocolate broken into chunks in a small pan and gently heat, stirring until completely melted.
Mix together the flour, sugar, cocoa and bicarbonate of soda with a pinch of salt in a mixing bowl. Whisk the eggs and buttermilk or yogurt together. Scrape the melted chocolate mixture and egg mixture into the dry ingredients, add 100ml boiling water and whizz briefly with an electric whisk until the cake batter is lump free.
Divide the mixture between the tins and bake for 20 minutes, swapping the tins round after 15 minutes if they’re on different shelves. To test they're done, push in a skewer and check that it comes out clean.
Prick the cakes a few times with a skewer. Mix together the reserved cherry juice and the brandy and drizzle over the cakes. Cool the cakes.
Mix together the remaining drained cherries and jam. Tip 300ml of the cream into a small pan and heat until just below simmering point. Chop the remaining chocolate and put in a heatproof bowl, pour over the hot cream and stir until melted. Set aside until spreadable.
When the cakes are cool whisk the remaining cream and the icing sugar together until softly whipped. Spread over two of the cakes, then spoon over the jammy cherries. Stack the cakes together. Spread the chocolate cream over the third cake and sit on top of the other cakes.