Black Forest gateau

Name: Black Forest gateau
Cook time: 40 min
Total time: 40 min
Cuisine: European, German
Ingredients: 175g salted butter, plus extra for greasing; 200g bar dark chocolate; 300g plain flour; 375g golden caster sugar; 25g cocoa; 1 tsp bicarbonate of soda; 2 medium eggs; 200g buttermilk or natural yoghurt; 425g can pitted cherry, 2 tbsp juice reserved, rest drained; 100g morello cherry jam; 4 tbsp kirsch (or more juice from a can if you want it to be non-alcoholic); 500ml tub double cream; 3 tbsp icing sugar; 1 small punnet fresh cherries (optional)