Heat oven to 180C/160C fan. To make the cupcakes, put the flour, cocoa powder, sugar, baking powder, a pinch of salt and the butter in a bowl and mix with an electric whisk. Beat on a slow speed until you get a sandy consistency, and everything is combined.
Whisk the milk, egg and vanilla extract together in a jug, then slowly pour about half into the flour mixture. Beat to combine and turn the mixer up to high speed, scraping any mixture from the side of the bowl. Pour in the remaining liquid and continue mixing for a couple more minutes until the mixture is smooth. Do not overmix.
Spoon the mixture into the paper cases until approximately two-thirds full. Bake in a preheated oven for 20-25 minutes, or until the sponge bounces back when touched and a skewer inserted into the centre comes out clean.
Leave the cupcakes to cool slightly in the tray before turning out onto a wire cooling rack to cool completely.
Use a sharp knife to cut a cone out of the centre of each cupcake. Set aside.
Melt the chocolate in a bowl until smooth. Set aside to cool slightly while you make the buttercream.
Beat sifted icing sugar and butter together until a sandy consistency, slowly poor in the cooled chocolate, add 50g of strawberry jam and beat until smooth. Add 2-3 tablespoons of boiling water, if necessary, to loosen.
To finish the cupcakes, fill the centres of each cupcake with 3/4 to 1 teaspoon of the remaining strawberry jam. Fill a piping bag with a decorative tip with the buttercream and pipe the buttercream of top of each cupcake.
Store the cupcakes in the refrigerator but let them come to room temperature for a few minutes before eating.