Problem with the Chocolate, Prune and Armagnac Cake

Having only recently, for the same reason as half the rest of the country, discovered the pleasure of baking cakes regularly (& so far amazingly successfully, including Delia’s divine Austrian Coffee & Walnut and Coffee & Cardamon cakes, both made 3 times now) my Waterloo has been Delia’s Chocolate, Prune & Armagnac cake. First of all the actual cake was definitely not soufflé-like, just rather dull cake. What did I do there? Secondly the chocolate topping was fine & glossy for dipping the prunes, but turned grainy & flat when I added the creme fraiche. So the cake looked nothing like your picture and was really only just edible. I did use the correct sponge tins, scrupulously clean bowl & whisk for the egg whites. Also Green & Black’s cocoa powder. The things I didn’t have were Armagnac, so used Brandy; I only had 200 g of pruneaux d’Agen so added 200g of M&S pitted prunes; and didn’t have 75% chocolate so used half 70% and half 85%. Sorry this is so rambling a question but I was so dispirited by this setback I took to drink. Thank you anyway