Cream 125 g icing sugar and butter with the whisk of the hand mixer. Stir in egg and vinegar. Mix flour, starch, baking soda, baking powder and cocoa. Alternately stir the flour mixture with the buttermilk into the fat mass.
Fill the dough into a greased springform pan (20 cm Ø), dust with flour and smooth it down. Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 35 minutes.
Remove and let it cool down on a cake rack.
Wash the lemon, grate dry and grate half of the peel. Drain the sour cherries, collecting the juice. Stir pudding powder with 4 tbsp. juice until smooth. Boil up the rest of the juice with 75 g sugar, lemon zest and rum in a pot.
Remove from heat and stir in the mixed starch. Simmer for about 1 minute while stirring. Mix in the cherries.
Carefully remove the base from the mould and cut in half horizontally. Place the bottom cake layer on a cake plate and enclose with a cake ring. Spread the cherries on top and place the upper cake layer on top. Chill the cake for about 3 hours.
Roughly chop the chocolate and melt over a warm water bath. Pour liquid chocolate into a small disposable piping bag, cut off the tip and spray onto a baking tray lined with baking paper with fir trees of different sizes.
Put them in a cool place until they have set.
Whisk the egg whites, salt and maple syrup over a warm water bath, slowly adding 100 g of sugar. When all the sugar is dissolved, remove the beaten egg whites from the water bath and continue beating for approx. 5 minutes.
Stir in 25 g icing sugar.
Remove the cake ring and coat the cake with the beaten egg white. Carefully remove fir trees from the baking paper and place them on the edge of the cake. Coarsely crumble the meringue and spread on the cake.