Preheat the oven to 150°C fan. Line a muffin tin with 12 cupcake cases.
Sieve the flour, cocoa powder, baking powder, bicarbonate of soda and salt into a large bowl.
In a separate large bowl, whisk together the sugar, oil, vanilla, egg and NI buttermilk for a couple of minutes with a balloon whisk.
Add the dry ingredients to the liquid and whisk until just combined. Then pour in the boiling water and carefully whisk together.
Pour the batter into the cupcake cases - I use an ice cream scoop as I find it the easiest. Only fill the cupcake cases halfway-do not be tempted to fill right up or they will overflow!
Bake for 16-18 mins until risen and a skewer comes out clean. Take the cupcakes out of the tins and place onto a cooling rack to completely cool.
For the icing, melt the chocolate in the microwave in 30 second intervals and leave aside to cool.
Mix the butter until soft and light in colour in a stand mixer (about 5 minutes). Add the sifted icing sugar and cocoa powder and mix on low until the sugar incorporates and then up to high speed for 2-3 minutes.
Add the vanilla, cooled melted chocolate and milk and mix until combined. Put into the fridge to cool for 30 minutes.
You can either spread on top of the cupcakes with a knife or put the buttercream into a piping bag fitted with a large star nozzle and pipe your buttercream on like I did.