Sieve the flour, cocoa, sugar, baking powder, salt and butter into the bowl of a KitchenAid (or other freestanding mixer) and using the paddle attachment mix slowly until everything is combined and you have a sandy consistency (if you don't have a KitchenAid use a hand-held electric beater).
While the mixer is running, slowly pour in half of the milk and mix until it's just incorporated. Add the egg to the remaining milk along with the vanilla and continue beating until it's mixed in. Scrape down the sides of the bowl to make sure everything is properly mixed but be careful not to over beat the mixture.
Evenly spoon the cake mixture into the cupcake cases and bake for 20-25 minutes until the sponge bounces back when touched (with my oven 22 minutes works perfectly every time) and a skewer comes out clean with put into the center of the cake.
Once the cupcakes have cooled scoop out a little of the center. I use an apple corer as I find it works perfectly, however you could easily just use a small knife or teaspoon. Fill the center of each cupcake with a teaspoon of cherry jam.
Once you are ready to serve the cupcakes put the cream, vanilla and sugar in a large bowl and whip until soft peaks that hold their shape. Spoon the cream onto each cupcake and swirl with a small spatula (or knife) then using a vegetable peeler shave over some chocolate before finishing with a cherry on top! The perfect Birthday treat!
The cupcakes can be made a day or two in advance, however the cream won't keep on them so only add the cream when you are ready to serve.