Preheat the oven to gas mark 4/180oC/350oF/160oC fan and line a 12 count muffin tin with paper cases. In a bowl, whisk the self-raising flour, caster sugar, cocoa powder and bicarbonate soda (baking soda) until combined then set aside.
In another bowl add and mix the eggs, sour cream, buttermilk, vanilla extract and vegetable oil. The mixture will look lumpy at this point. Pour the dry ingredients into the wet ingredients and whisk until smooth and lump free. Spoon the mixture into the cupcake cases and bake for 23 minutes or until a cake tester inserted into the centre comes out clean. Transfer the cupcakes to a wire rack to cool.
For the ganache: add the dark chocolate to a mixing bowl and set aside. In a saucepan, heat the double cream and sugar and bring to a boil. Ensure you do not leave the cream unattended as it will boil over.
Pour the hot cream over the chocolate and ensure all the chocolate is submerged into the cream. Allow to stand for 3 minutes to allow the chocolate to melt then stir the mixture for about 4 minutes or until all the mixture is fully combined and the chocolate is smooth and glossy.
Allow to completely cool then transfer the ganache to a stand mixer and whisk until the ganache lightens in colour. You can also use a hand mixer. Once whipped, spoon the ganache into a piping bag with your desired nozzle and pipe the ganache onto the cupcakes.