Preheat the oven to 175c (350f) and line a 12-hole muffin tin with black cupcake liners.
Whisk the vegetable oil and caster sugar together in a large bowl until creamy (about 2 minutes with an electric whisk). Add the eggs, vanilla extract and buttermilk and stir to combine.
Sieve in the cocoa powder, flour and bicarbonate of soda. Fold in using a spatula. Add a pinch of salt and the black food colouring gel. Fold again until combined. You may need to add more colouring, but it's worth noting that the mix will darken as it cooks in the oven. The mixture will be very runny. Spoon the mixture into the cupcake liners - approx 2/3rds full.
Place in the oven and bake for 18-20 minutes, until an inserted skewer comes out clean. Leave to cool for 10 minutes in the tray, then transfer to a baking rack to cool completely.
Whilst the cakes are cooling, make the filling. Pour the cherries in kirsch into a saucepan and heat until just simmering. Stir in a little of the cornflour mixture to thicken. Add more if needed to get a mixture that is the thickness of cherry pie filling. Turn off the heat and leave to cool.
Next make the buttercream. Melt the chocolate in the microwave in 30 second bursts, stirring in between each burst (or melt over a bain marie if you prefer). Leave to cool to room temperature.
Place the butter, confectioners' sugar, vanilla extract and salt in a large bowl and whisk, using an electric whisk until thick and smooth. Add the cooled, melted chocolate and whisk again. If the mixture is too thick for piping onto a cake, add the milk, a tiny drizzle at a time and whisk, until you get the right consistency.
Whisk in the food colouring gel until you get the colour you want. Spoon the buttercream into an icing bag that has a wide fluted icing nozzle.
Once the cupcakes have cooled, use a teaspoon to scoop out one heaped teaspoon of cake from the centre (no more than this or the cakes will collapse). Fill the hole with the cherry mixture - I used 2-3 cherries per cake.
Pipe the buttercream on, starting from the middle, working your way out, then back into the centre again, to get a dome-shaped buttercream topping. Repeat with the remaining cupcakes.
Place a cherry on top of each cupcake and serve!